Fix repeater holding onto elements

If a consumer changes the ItemsSource with a non recycling layout then the items from the old source would be perpetually held by repeater, a potentially substantial leak.

Ported from e158eec3e9

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -588,6 +588,8 @@ namespace Avalonia.Controls
                             ClearElementImpl(element);
                         }
                     }
+
+                    Children.Clear();
                 }
 
                 InvalidateMeasure();