Add systemd units
So that we can be started by systemd --user, instead of gnome-session.
There are three units:
- gnome-shell.service: Start gnome-shell itself.
- gnome-shell-x11.target, gnome-shell-wayland.target: Sync points for units that need to care if x11 or wayland is in use. gnome-settings-daemon will use these, for example.
This goes with !137 (merged). It's safe to merge this at any point, as the units don't do anything until something starts them.
There's a small chance the structure will change if we find a problem, but we'll be able to fix it up then if that happens.