If set for a widget, when a drop occurs, GTK+ will will check if the drag matches this widget’s list of possible targets and actions.
If so, GTK+ will call drag_get_data on behalf of the widget. Whether or not the
drop is successful, GTK+ will call drag_finish. If the action was a move, then if the
drag was successful, then true will be passed for the delete parameter to
drag_finish.