This makes sure other packaging formats get the configuration too.

Install it to /usr/share/gnome-session/ so GNOME's gnome-session
can find it.

gnome-session starts applications by desktop file.

We set X-GNOME-AutoRestart=false for the moment to make it simpler to
spawn new development versions. We might change that at a later point
for release builds.

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

(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.