screenShield: Raise lightbox on blank

screenShield: Raise lightbox on blank

The lightboxes used for screen blanking are created during initialization
and shown each time the screen should blank. During the (potentially long)
time where the lightbox is hidden, any actor could be raised above the
lightbox - in particular any popup menu raises itself to the top when
opened. To not exclude those elements from screen blanking, raise the
lightbox every time it is shown.

https://bugzilla.gnome.org/show_bug.cgi?id=773875