Commit d0310bd745968 blindly replaced global.overlay_group with
Main.layout.overviewGroup, but unlike the former, the latter is
hidden while the overview is not active, which makes it unsuitable
for the message tray's light box. In fact, with the removal of
global.overlay_group, there is no longer a container which may
be used both inside and outside the overview, so we can either
recreate the lightbox each time we show/hide the overview, or
use different lightboxes altogether; this opts for the latter.
https://bugzilla.gnome.org/show_bug.cgi?id=701097
365cda386c