Register TabControl with ContentControlMixin.

`ContentControlMixin` implements a number of behaviors related to logical children that controls that have a `ContentPresenter` need in order to function correctly.

Also renamed `PART_Content` to `PART_SelectedContentHost` to match WPF

Fixes #2191

samples/ControlCatalog/SideBar.xaml

@@ -26,7 +26,7 @@
                             </ItemsPresenter>
                         </ScrollViewer>
                         <ContentPresenter
-                            Name="PART_Content"
+                            Name="PART_SelectedContentHost"
                             Margin="{TemplateBinding Padding}"                           
                             HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                             VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"

src/Avalonia.Controls/TabControl.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using Avalonia.Controls.Generators;
+using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
@@ -65,6 +66,10 @@ namespace Avalonia.Controls
             SelectionModeProperty.OverrideDefaultValue<TabControl>(SelectionMode.AlwaysSelected);
             ItemsPanelProperty.OverrideDefaultValue<TabControl>(DefaultPanel);
             AffectsMeasure<TabControl>(TabStripPlacementProperty);
+            ContentControlMixin.Attach<TabControl>(
+                SelectedContentProperty,
+                x => x.LogicalChildren,
+                "PART_SelectedContentHost");
         }
 
         /// <summary>
@@ -142,7 +147,7 @@ namespace Avalonia.Controls
 
             ItemsPresenterPart = e.NameScope.Get<ItemsPresenter>("PART_ItemsPresenter");
 
-            ContentPart = e.NameScope.Get<ContentPresenter>("PART_Content");
+            ContentPart = e.NameScope.Get<ContentPresenter>("PART_SelectedContentHost");
         }
 
         /// <inheritdoc/>

src/Avalonia.Themes.Default/TabControl.xaml

@@ -18,7 +18,7 @@
                             MemberSelector="{TemplateBinding MemberSelector}"                     >
                         </ItemsPresenter>
                         <ContentPresenter
-                            Name="PART_Content"
+                            Name="PART_SelectedContentHost"
                             Margin="{TemplateBinding Padding}"
                             HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                             VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -302,7 +302,7 @@ namespace Avalonia.Controls.UnitTests
                         },
                         new ContentPresenter
                         {
-                            Name = "PART_Content",
+                            Name = "PART_SelectedContentHost",
                             [!ContentPresenter.ContentProperty] = parent[!TabControl.SelectedContentProperty],
                             [!ContentPresenter.ContentTemplateProperty] = parent[!TabControl.SelectedContentTemplateProperty],
                         }