introduce WidgetMoverButton (to realize the "move layer" feature in the upcoming layer oriented sample editor)

Add the possibility to show the empty widget in ItemComboWidget also for other items (e.g. running jobs)

Add a blank widget to ItemComboWidget for the case that a nullptr SessionItem is set as the current item (instead of the show/hide logic)

Merge remaining funcionality of JobOutputDataWidget into JobResultsPresenter and delete obsolete JobOutputDataWidget

Remove ItemStackPresenter (which was only used in "single widget mode" anyway) from JobOutputDataWidget