data: Ensure shutdown target does not keep units loaded

The shutdown target has explicit conflicts on some user units, this
means that as long as it is active, it will keep those units loaded.

We could probably reverse the conflicts to prevent this from happening.
But, it makes sense to set StopWhenUnneeded=true anyway and doing so
also means that the corresponding units can be unloaded after logout.

data/gnome-session-shutdown.target

@@ -16,7 +16,11 @@ After=gnome-session.target gnome-session-manager.target
 Conflicts=gnome-session-pre.target gnome-session-initialized.target gnome-session-failed.target
 After=gnome-session-pre.target gnome-session-initialized.target gnome-session-failed.target
 
-# Could we set StopWhenUnneeded=true or would that prevent the service to start?
+# We need to make sure this unit is stopped; primarily so that the tree of
+# units that we created is completely cleaned.
+# Note that this can also be improved by reversing the conflicts above and
+# not listing them in the shutdown unit.
+StopWhenUnneeded=true
 
 # We trigger a restart of DBus after reaching the shutdown target this
 # is a workaround so that DBus services that do not connect to the