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>

Hosting/LinuxMauiAppBuilderExtensions.cs

@@ -98,6 +98,9 @@ public static class LinuxMauiAppBuilderExtensions
             handlers.AddHandler<ImageButton, ImageButtonHandler>();
             handlers.AddHandler<GraphicsView, GraphicsViewHandler>();
 
+            // Web
+            handlers.AddHandler<WebView, WebViewHandler>();
+
             // Collection Views
             handlers.AddHandler<CollectionView, CollectionViewHandler>();
             handlers.AddHandler<ListView, CollectionViewHandler>();