The xdg_session_manager_v1 global interface is the generator
of xdg_session_v1 objects for clients. These will notify of an
unique ID that can be used for future instantiations.
Once a xdg_session_v1 object is obtained, toplevels can be added
to be managed by it, and clients may get a hint about whether the
toplevel was restored to a saved state.
Changes by Carlos Garnacho: Integrate with MetaSessionManager core
object. Flesh out event emission of xdg_session_v1 and
xdg_toplevel_session_v1 objects, handle sessions being
replaced/deleted.
Changes by Sebastian Wick:
* make lifetimes of xdg_sessions entirely determined by the wayland and
handle its destruction via the signal
* fix session destruction vs deletion
* do not drop refcount of replaced session state temporarily to make
sure the replacing session keeps the state
* disconnect signals of destroyed and replaced sessions
* disconnect window-unmanaging signal handler for
MetaWaylandXdgToplevelSession
* call wl_resource_destroy in xdg_toplevel_session_remove to make it a
destructor
* handle session being destroyed before topevel-sessions
* handle the toplevel going away before the topevel-sessions
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3825>
