Fix a bug where drag and dropping a folder in the model tree could orphan diagram elements

* The logic in TreeModelViewerDragDropHandler was wrong
* If you dragged and dropped onto its existing parent folder it got added to the elements child list
* This caused the folder to be saved as an element not a folder type
* If this happened to a sub-folder in Views and you then deleted any elements from the model tree they would be orphaned in the View