Add WebView support via WebKitGTK

Implements Priority 4 item: WebView via WebKitGTK

New files:
- Interop/WebKitGtk.cs - P/Invoke bindings for WebKitGTK library
- Views/LinuxWebView.cs - WebKitGTK-based WebView platform control
- Handlers/WebViewHandler.Linux.cs - MAUI handler for WebView on Linux
- samples_temp/WebViewDemo/ - Demo app for WebView functionality

Features:
- Full HTML5/CSS3/JavaScript support via WebKitGTK
- Navigation (back/forward/reload)
- URL and HTML source loading
- JavaScript evaluation
- Navigation events (Navigating/Navigated)
- Automatic GTK event processing

Requirements:
- libwebkit2gtk-4.1-0 package on target Linux system

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

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

samples_temp/ShellDemo/Platforms/Linux/Program.cs

@@ -0,0 +1,19 @@
+// Platforms/Linux/Program.cs - Linux platform entry point
+// Same pattern as Android's MainActivity or iOS's AppDelegate
+
+using Microsoft.Maui.Platform.Linux;
+using Microsoft.Maui.Platform.Linux.Hosting;
+
+namespace ShellDemo;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        // Create the shared MAUI app
+        var app = MauiProgram.CreateMauiApp();
+        
+        // Run on Linux platform
+        LinuxApplication.Run(app, args);
+    }
+}