SwitcherPopup relies on being able to pushModal(), setting the stage
input mode to FULLSCREEN, and then doing regular event processing on
the actor it adds to uiGroup. But MessageTray uses GrabHelper which
sets up a 'captured-event' handler on the stage and thus gets all
events itself.
This, of course, breaks the switcher if it's brought up in the message
tray so, for now, we'll just prevent it from being used there.
https://bugzilla.gnome.org/show_bug.cgi?id=693907
e5ca53e56c