housekeeping: Add systemd OOM notification module

[Fantu]

Notify the user when systemd kills a unit (scope/service) due to an out-of-memory event. The notification reports the application name when the desktop entry can be resolved from the cgroup unit name, with a generic fallback for virtual terminals and unknown units.

Note that we only notify if a unit (scope/service) is actually stopped due to OOM. This will not catch certain other cases where e.g. a delegated cgroup receives an OOM event.

Ported from gnome-settings-daemon, squashing the following upstream commits:

d1fe7167 housekeeping: Add systemd OOM notification module
fb4edd88 housekeeping: fix capitalization of systemd notifications
e6c41410 Use the proper hint for setting images for notifications
dfb7aeab housekeeping: Fix wording in the OOM kill message
79d4fc90 housekeeping: Fix systemd OOM notifications
(call org.freedesktop.systemd1.Manager.Subscribe so that
PropertiesChanged signals are actually emitted)

Adaptations vs. upstream:

• Renamed symbols/files to the cinnamon-settings-daemon prefixes (gsd -> csd, Gsd -> Csd, GSD -> CSD).
• The new CsdSystemdNotify instance is held in the private struct CsdHousekeepingManagerPrivate, since CsdHousekeepingManager still uses g_type_class_add_private rather than inline struct fields like the upstream GsdHousekeepingManager.

[Fantu]

I think is useful for users that want use systemd-oomd and for distro that use it by default like Fedora and Ubuntu.

I haven't tested it yet, and I probably won't have time to do so until next weekend.