Add a framework for restarting the compositor with nice visuals

The current GNOME Shell Alt-F2 restart looks very messy and also
provides no indication to the user what is going on. We need to
restart the compositor to switch in and out of stereo mode, so
add a framework for doing this more cleanly:

Additions:

 meta_restart(): restarts the compositor with a message
 MetaDisplay::show-restart-message: signal the embedding
    shell to show a message
 MetaDisplay::restart: signal the embedding shell to restart
    itself.
 meta_is_restart(): indicates whether the current instance is a
                    restart so we can suppress login animations.

A helper program meta-restart-helper holds the composite overlay
window up during the restart to avoid visual artifacts.

https://bugzilla.gnome.org/show_bug.cgi?id=733026