Draft: systemd transient scope

This one also works with the stupid stuff that gnome-session does (compared to breaking it if we try to use transient services).

For context, I first tried transient service units !1592 (closed), which is the superior approach in general.

EDIT: I thought I had seen empty scopes earlier, but that doesn't seem to be true. Not sure what happened, maybe something kept them alive (e.g. a bug in the wrapper glib script). Either way, seems resolved.