Focus prefab window on item spawn.

Source/Editor/Windows/Assets/PrefabWindow.Hierarchy.cs

@@ -450,6 +450,7 @@ namespace FlaxEditor.Windows.Assets
             // Create undo action
             var action = new CustomDeleteActorsAction(new List<SceneGraphNode>(1) { actorNode }, true);
             Undo.AddAction(action);
+            Focus();
             Select(actorNode);
         }