appIcon: End running drag operations on destroy

appIcon: End running drag operations on destroy

AppIcon makes itself draggable, and handles the various DnD
routines such as 'drag-begin' and 'drag-end' by making the
Overview emit the appropriate signals.

However, when destroyed, the AppIcon does not try to finish
any drag operations that started. That causes the event
blocker in AllView not to be updated correctly when dragging
icons to outside folders.

Make AppIcon emit 'item-drag-end' when a drag operation
started and it's destroyed.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/643