core: Make MetaContext manage the MetaSessionManager singleton

Make this core object own the MetaSessionManager, for the window management
code to access.

At this level, we will be able to integrate with systemd notification
system, and use systemd fdstore to keep the mapped memory warm for
us for the case of soft reboot. This is at the moment not implemented
here.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3825>

src/core/meta-context.c

@@ -979,3 +979,9 @@ meta_context_get_debug_control (MetaContext *context)
 
   return priv->debug_control;
 }
+
+MetaSessionManager *
+meta_context_get_session_manager (MetaContext *context)
+{
+  return META_CONTEXT_GET_CLASS (context)->get_session_manager (context);
+}