Add Actor.HasScene and Script.HasScene

Useful in managed code to check the existence of the scene in hot paths
by avoiding expensive marshalling of the Scene object.

Source/Editor/Content/Proxy/PrefabProxy.cs

@@ -89,7 +89,7 @@ namespace FlaxEditor.Content
                 // Cleanup it after usage
                 Object.Destroy(actor, 20.0f);
             }
-            else if (actor.Scene != null)
+            else if (actor.HasScene)
             {
                 // Create prefab with identity transform so the actor instance on a level will have it customized
                 resetTransform = true;