On various events, it is convenient or even necessary for XDarwin-fullscreen
to switch back to Quartz, but XDarwin does not do it:

- when an application on the Quartz side requests attention
- when I run "open http://foo/bar" from an xterm to start Safari
- before going to sleep or before Quartz does an auto-lock-screen

This last one is important. As it stands, if I'm in XDarwin-fullscreen the
auto-lock-screen of Quartz never seems to kick-in, and if I put the machine to
sleep, when I wake up I'm straight back in XDarwin although the Quartz side
has actually popped up a dialog box requesting my password.