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
3.10.3