Preview 3: Complete control implementation with XAML data binding

Major milestone adding full control functionality:

Controls Enhanced:
- Entry/Editor: Full keyboard input, cursor navigation, selection, clipboard
- CollectionView: Data binding, selection highlighting, scrolling
- CheckBox/Switch/Slider: Interactive state management
- Picker/DatePicker/TimePicker: Dropdown selection with popup overlays
- ProgressBar/ActivityIndicator: Animated progress display
- Button: Press/release visual states
- Border/Frame: Rounded corners, stroke styling
- Label: Text wrapping, alignment, decorations
- Grid/StackLayout: Margin and padding support

Features Added:
- DisplayAlert dialogs with button actions
- NavigationPage with toolbar and back navigation
- Shell with flyout menu navigation
- XAML value converters for data binding
- Margin support in all layout containers
- Popup overlay system for pickers

New Samples:
- TodoApp: Full CRUD task manager with NavigationPage
- ShellDemo: Comprehensive control showcase

Removed:
- ControlGallery (replaced by ShellDemo)
- LinuxDemo (replaced by TodoApp)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Services/SystemClipboard.cs

@@ -0,0 +1,248 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+
+namespace Microsoft.Maui.Platform;
+
+/// <summary>
+/// Static helper for system clipboard access using xclip/xsel.
+/// Provides synchronous access for use in UI event handlers.
+/// </summary>
+public static class SystemClipboard
+{
+    /// <summary>
+    /// Gets text from the system clipboard.
+    /// </summary>
+    public static string? GetText()
+    {
+        // Try xclip first
+        var result = TryGetWithXclip();
+        if (result != null) return result;
+
+        // Try xsel as fallback
+        result = TryGetWithXsel();
+        if (result != null) return result;
+
+        // Try wl-paste for Wayland
+        return TryGetWithWlPaste();
+    }
+
+    /// <summary>
+    /// Sets text to the system clipboard.
+    /// </summary>
+    public static void SetText(string? text)
+    {
+        if (string.IsNullOrEmpty(text))
+        {
+            ClearClipboard();
+            return;
+        }
+
+        // Try xclip first
+        if (TrySetWithXclip(text)) return;
+
+        // Try xsel as fallback
+        if (TrySetWithXsel(text)) return;
+
+        // Try wl-copy for Wayland
+        TrySetWithWlCopy(text);
+    }
+
+    private static string? TryGetWithXclip()
+    {
+        try
+        {
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "xclip",
+                Arguments = "-selection clipboard -o",
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return null;
+
+            var output = process.StandardOutput.ReadToEnd();
+            process.WaitForExit(1000);
+
+            return process.ExitCode == 0 ? output : null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    private static string? TryGetWithXsel()
+    {
+        try
+        {
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "xsel",
+                Arguments = "--clipboard --output",
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return null;
+
+            var output = process.StandardOutput.ReadToEnd();
+            process.WaitForExit(1000);
+
+            return process.ExitCode == 0 ? output : null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    private static string? TryGetWithWlPaste()
+    {
+        try
+        {
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "wl-paste",
+                Arguments = "--no-newline",
+                UseShellExecute = false,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return null;
+
+            var output = process.StandardOutput.ReadToEnd();
+            process.WaitForExit(1000);
+
+            return process.ExitCode == 0 ? output : null;
+        }
+        catch
+        {
+            return null;
+        }
+    }
+
+    private static bool TrySetWithXclip(string text)
+    {
+        try
+        {
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "xclip",
+                Arguments = "-selection clipboard",
+                UseShellExecute = false,
+                RedirectStandardInput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return false;
+
+            process.StandardInput.Write(text);
+            process.StandardInput.Close();
+
+            process.WaitForExit(1000);
+            return process.ExitCode == 0;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    private static bool TrySetWithXsel(string text)
+    {
+        try
+        {
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "xsel",
+                Arguments = "--clipboard --input",
+                UseShellExecute = false,
+                RedirectStandardInput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return false;
+
+            process.StandardInput.Write(text);
+            process.StandardInput.Close();
+
+            process.WaitForExit(1000);
+            return process.ExitCode == 0;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    private static bool TrySetWithWlCopy(string text)
+    {
+        try
+        {
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "wl-copy",
+                UseShellExecute = false,
+                RedirectStandardInput = true,
+                RedirectStandardError = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process == null) return false;
+
+            process.StandardInput.Write(text);
+            process.StandardInput.Close();
+
+            process.WaitForExit(1000);
+            return process.ExitCode == 0;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    private static void ClearClipboard()
+    {
+        try
+        {
+            // Try xclip
+            var startInfo = new ProcessStartInfo
+            {
+                FileName = "xclip",
+                Arguments = "-selection clipboard",
+                UseShellExecute = false,
+                RedirectStandardInput = true,
+                CreateNoWindow = true
+            };
+
+            using var process = Process.Start(startInfo);
+            if (process != null)
+            {
+                process.StandardInput.Close();
+                process.WaitForExit(1000);
+            }
+        }
+        catch
+        {
+            // Ignore errors when clearing
+        }
+    }
+}