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/WebViewDemo/WebViewDemo.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RootNamespace>WebViewDemo</RootNamespace>
+    <RuntimeIdentifier>linux-arm64</RuntimeIdentifier>
+    <SelfContained>true</SelfContained>
+    <PublishSingleFile>false</PublishSingleFile>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="../../OpenMaui.Controls.Linux.csproj" />
+  </ItemGroup>
+
+</Project>