Seal OnPropertyChangedCore for now.

It's only likely to be needed by transitions, and overriding it will break transitions. We can unseal later if there's a user need for it.

src/Avalonia.Animation/Animatable.cs

@@ -116,7 +116,7 @@ namespace Avalonia.Animation
             }
         }
 
-        protected override void OnPropertyChangedCore<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected sealed override void OnPropertyChangedCore<T>(AvaloniaPropertyChangedEventArgs<T> change)
         {
             if (_transitionsEnabled &&
                 _transitions is object &&