This is an abstract base class to implement a "client session",
carrying the accounting of the windows, and allowing to serialize/read
their state into a Gvdb table.
Since different windowing backends may require slightly different
data to be saved for each window, this is meant to have windowing-specific
implementations.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3825>
