[ initially investigated in bug 693907 ]

The problem is that 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.

The only way I can see to make this work is having a global GrabHelper instance
and port every use of pushModal() to use that instance instead. It goes without
saying that I don't feel comfortable doing that at this point in the release
cycle.

Any other suggestions?