Add support for meta_restart() and MetaDisplay::restart

Support was added to Mutter to allow it to trigger a restart
to allow for restarts when switching in or out of stereo mode.

Hook up to the new signals on MetaDisplay to show the restart
message and reexec. Meta.is_restart() is used to suppress
the startup animation.

This also allows us to do 'Alt-F2 r' restarts more cleanly
without a visual flash and animation.

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