Warn when user is removing an element that is referenced elsewhere in QML

While in Live Preview, the user can currenly delete any sort of item. When saving the file, delta detects this and requests deletion from the running qml engine. While everything might work well while running the application, the next time user runs can result in errors if the deleted element is referenced elsewhere. Because of this, LiveTextPreview/UpdateObserver classes should check that when an item is being deleted, and issue a warning in the editor if an element is being referenced elsewhere.