Move the GNOME shell service file adapation for x11/wayland into the
target/service files. This means that the session definition can simply
pull in org.gnome.Shell.target, without having to care about whether it
is starting an X11 or wayland session.
Note that this currently requires fork'ing to do the test. This will
however not be needed in the long term when ConditionEnvironment becomes
available (see https://github.com/systemd/systemd/pull/15817).
We technically do not need to use template units. But doing so means
that the unit can be translated to the app id more easily (though it is
not yet completely clear how this should look like in the long term).
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/895
