Allow saving the session gvdb file in the background, with as little
overhead in the main thread as possible. We still need to serialize
all created/deserialized MetaSessionState to a GVDB hashtable there,
in order to avoid these being poked from the async task thread.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3825>
This core object will be the manager of "client sessions", allowing
the windowing-specific paths to generate MetaSessionState objects to
track their clients.
This object is unused at the moment, and will be integrated in later
commits.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3825>