This object is a windowing-specific implementation of MetaSessionState,
allowing to save window state for toplevel surfaces of a Wayland client
using the xdg_session_management_v1 protocol.
This object is detached from windowing logic itself, and will be
integrated in later commits.
Changes from Carlos Garnacho: Integrate state serialization with
MetaSessionState and MetaSessionManager.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3825>
