qml: fix tool bar editor delegate flickering

Currently, when drop() is called, the delegate is shown in its original position with internal drag. This causes the delegate to be shown for a momentary period of time until it is removed from the view.

In order to prevent flickering, hide the delegate before calling drop() and if the delegate is dropped in the same view (intra-view move) then show it since the delegate should not remain hidden in its new position.

Request review @chub.