Unify the style of <delegate>.destroy() methods to only contain a
call to <delegate>.actor.destroy() and handle additional cleanup
in a _onDestroy() signal handler.
https://bugzilla.gnome.org/show_bug.cgi?id=609454
Make the event blocking done by Lightbox optional - leave it activated
when displaying the run dialog, but deactivate it when highlighting
windows in the overview.
https://bugzilla.gnome.org/show_bug.cgi?id=602774