When a non-client app with AutoRestart enabled exits immediately after
starting, the restart logic would loop without delay, causing high CPU
usage. This adds a 60-second rate limit to gsm_app_restart. If an app
respawns twice within this window, the second restart is rejected.

The code is adapted from:
- https://gitlab.gnome.org/GNOME/gnome-session/-/commit/4df48234
- https://gitlab.gnome.org/GNOME/gnome-session/-/commit/b9aa675e

Fixes #336

Things like XDG_SESSION_ID should not be uploaded to the environment.
For example this is broken currently:

  1. SSH to your machine
  2. Log in to MATE Shell
  3. Log out
  4. Log in again
  5. Lock the screen
  6. Try to unlock

You can't, and this is because the XDG_SESSION_ID from the first session
(step 2) has leaked through to the second one (step 4), and so MATE
Shell is listening to the `logind` `UnlockSession` signal for the wrong
session. The SSH session established in step 1 serves to keep the
`systemd --user` instance alive, so that the state is not torn down
between logins.

Also, The NOTIFY_SOCKET environment variable was leaking into systemd
managed MATE sessions and breaking things like OCI container runtimes
(eg., runc and crun). This variable is absent in non-systemd
managed sessions.

Backported from:
- https://gitlab.gnome.org/GNOME/gnome-session/-/commit/646b9bc0
- https://gitlab.gnome.org/GNOME/gnome-session/-/commit/9d8b0709

Fixes #271

Apps like Orca set X-GNOME-AutoRestart=true in their .desktop files
expecting to be restarted after a crash. However, the restart logic only
handled apps that registered as XSMP or D-Bus clients. Non-client apps
emitted "died"/"exited" signals but nothing subscribed to them. This bug
was inherited from gnome-session.

Connect the "died" and "exited" signals on autostart apps to trigger
restart for non-client apps with AutoRestart enabled. Also fall back to
X-GNOME-AutoRestart when X-MATE-AutoRestart is not present, for
compatibility with apps that only set the GNOME key.

Fixes #321

The desktop-logout sound event was being played during the
query-end-session phase, which means it would play even if an
application inhibited the logout and the user cancelled. Move it to the
end-session phase where logout is already confirmed.

This reverts commit ea99b5fef1522b2fdc8aa52b2f6ccf2920d25b67.