messageTray: Don't show non urgent notifications while in fullscreen

Don't show non urgent notifications when the primary monitor
is in fullscreen (user watching a movie, having a presentation,
playing a game ...).

Once the user leaves fullscreen show the messagetray so that the user don't
miss any notification (same as in "back from idle").

https://bugzilla.gnome.org/show_bug.cgi?id=677590