Use X11 backend for Linux by default

samples/ControlCatalog.NetCore/Program.cs

@@ -43,7 +43,7 @@ namespace ControlCatalog.NetCore
         /// This method is needed for IDE previewer infrastructure
         /// </summary>
         public static AppBuilder BuildAvaloniaApp()
-            => AppBuilder.Configure<App>().UseX11().UseSkia().UseReactiveUI();
+            => AppBuilder.Configure<App>().UsePlatformDetect().UseSkia().UseReactiveUI();
 
         static void ConsoleSilencer()
         {

src/Avalonia.Desktop/AppBuilderDesktopExtensions.cs

@@ -29,7 +29,7 @@ namespace Avalonia
             }
             else
             {
-                LoadGtk3(builder);
+                LoadX11(builder);
                 LoadSkia(builder);
             }
             return builder;
@@ -42,9 +42,9 @@ namespace Avalonia
             where TAppBuilder : AppBuilderBase<TAppBuilder>, new()
              => builder.UseWin32();
 
-        static void LoadGtk3<TAppBuilder>(TAppBuilder builder)
+        static void LoadX11<TAppBuilder>(TAppBuilder builder)
             where TAppBuilder : AppBuilderBase<TAppBuilder>, new()
-             => builder.UseGtk3();
+             => builder.UseX11();
 
         static void LoadDirect2D1<TAppBuilder>(TAppBuilder builder)
             where TAppBuilder : AppBuilderBase<TAppBuilder>, new()

src/Avalonia.Desktop/Avalonia.Desktop.csproj

@@ -7,8 +7,8 @@
       <ProjectReference Include="../../src/Windows/Avalonia.Win32/Avalonia.Win32.csproj" />
       <ProjectReference Include="../../src/Windows/Avalonia.Direct2D1/Avalonia.Direct2D1.csproj" />
       <ProjectReference Include="../../src/Skia/Avalonia.Skia/Avalonia.Skia.csproj" />
-      <ProjectReference Include="../../src/Gtk/Avalonia.Gtk3/Avalonia.Gtk3.csproj" />
       <ProjectReference Include="../../src/Avalonia.Native/Avalonia.Native.csproj" />
       <ProjectReference Include="../../packages/Avalonia/Avalonia.csproj" />
+      <ProjectReference Include="../Avalonia.X11/Avalonia.X11.csproj" />
   </ItemGroup>
 </Project>