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.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/138
