systemd scopes

This will solve various issues:

• Killing of other processes when gnome-shell is restarted (X11)
• OOM behaviour without the need to set the OOMPolicy

Still missing here:

• Some error paths
• A drop-in unit to set defaults; or adding defaults directly from code (see gnome-session!32 (merged))
• RunDialog handling, as that uses a different code path
• Testing (the g-s-d one works)

See also gnome-settings-daemon!147 (merged)