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

...instead of MainWindow

this fixes #103 (GUI: "More..." button in JobWidget is not working)

explanation for changes: filling toolbars with buttons breaks the "more"-button functionality, therefore use the already existing actions to fill the toolbar

Moved some widgets from GUI/Views/JobWidgets to GUI/View/FitWidgets because they are (mainly) used there

Moved some widgets from GUI/Views/FitWidgets to GUI/View/JobWidgets because they are (mainly) used there