session: Add a drop-in for the application scopes cinnamon creates

[Fantu]

With linuxmint/cinnamon-desktop#275, cinnamon puts every application it launches into a transient app-cinnamon-*.scope. Those scopes are created with nothing but a PID list, so systemd knows nothing about their lifetime: at logout they are torn down abruptly when the user instance goes away, and if the user has lingering enabled they are not torn down at all.

This ships the same drop-in gnome-session ships for its own scopes (data/app-override.scope.conf, installed into app-gnome-.scope.d/):

• PartOf=graphical-session.target ties the applications to the session, so they are stopped when it ends instead of being killed when the user instance goes down.
• TimeoutStopSec=5s gives them a moment to exit cleanly first.
• CollectMode=inactive-or-failed keeps failed units from piling up.

This relies on graphical-session.target being activated by the session, which has been the case since e01c8c3 (#204).

Testing

Mint 23, Cinnamon 6.7.4 with the cinnamon-desktop fix applied — the drop-in is picked up by the scopes as they are created:

$ systemctl --user show app-cinnamon-debian-xterm-18470.scope \
      -p PartOf -p CollectMode -p TimeoutStopUSec -p Slice
PartOf=graphical-session.target
CollectMode=inactive-or-failed
TimeoutStopUSec=5s
Slice=app.slice

One caveat worth stating

graphical-session.target belongs to the systemd user instance, which is per user, not per session. If the same user has two graphical sessions running at once, logging out of one stops the application scopes of the other as well. Before application scopes existed the applications lived in their own session-N.scope and were unaffected. This is the same trade-off GNOME makes, and the multi-session case is rare, but it is a real behaviour change.