gnome-session needs an X11 display so start rootston first and make it
spawn gnome-session (which will spawn phosh later on).

This is more convenient for more complex defines

Fixes #7 (when there's also gnome-terminal installed)

This enables us to use translations

(based on gnome-shell/maynard)

We switch to a private shell protocol from here on.  This might get
folded back into weston's desktop shell protocol at a later point when
we see what else is needed.