This will be mainly useful for closing apps from the applications
switcher, but can be implemented generically enough to select the
nearest existing item after removal if there's any, or destroying
the popup's actor otherwise.
Specifically for the apps switcher, doing this also removes the need
of having to manually either update the current app in AppSwitcher
and highlight it, if there are still any items after the removal, or
simply destroy the AppSwitcher otherwise. Besides, calling _select()
in the handler for item-removed makes sure that the list of thumbnails
in the switcher is always closed, if open, when quitting the app.
https://bugzilla.gnome.org/show_bug.cgi?id=620106
06f790d86c