Only update selection if focus comes from TabItem. (#16338)

This is a temporary workaround to help Actipro's issues #15433: they're embedding other controls in a `TabControl`-derived control and they don't want these other controls to change the selection when they're focused.

src/Avalonia.Controls/TabControl.cs

@@ -256,7 +256,7 @@ protected override void OnGotFocus(GotFocusEventArgs e)
         {
             base.OnGotFocus(e);
 
-            if (e.NavigationMethod == NavigationMethod.Directional)
+            if (e.NavigationMethod == NavigationMethod.Directional && e.Source is TabItem)
             {
                 e.Handled = UpdateSelectionFromEventSource(e.Source);
             }