main: Unown our DBus name when gnome-session says "Stop"
We don't want to quit yet because if we do, gnome-shell and still mapped windows lose their theme and icons. But we have to unown our DBus name otherwise gnome-session will hang waiting for us. This only works due to a bug in gnome-session where it handles any client name being unowned as if the client has disconnected. Will need to be revisited when that bug is fixed in gnome-session. https://bugzilla.gnome.org/show_bug.cgi?id=727049