Enable transitions for non-Visuals.

`Visual` knows when it's attached to the visual tree, so it can enable transitions then.

This is a hack due to the fact that non-Visuals currently don't know when to enable transitions.

src/Avalonia.Animation/Animatable.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Animation
                 o => o.Transitions,
                 (o, v) => o.Transitions = v);
 
-        private bool _transitionsEnabled;
+        private bool _transitionsEnabled = true;
         private Transitions? _transitions;
         private Dictionary<ITransition, TransitionState>? _transitionState;
 

src/Avalonia.Visuals/Visual.cs

@@ -114,6 +114,9 @@ namespace Avalonia
         /// </summary>
         public Visual()
         {
+            // Disable transitions until we're added to the visual tree.
+            DisableTransitions();
+
             var visualChildren = new AvaloniaList<IVisual>();
             visualChildren.ResetBehavior = ResetBehavior.Remove;
             visualChildren.Validate = visual => ValidateVisualChild(visual);