x11/session: Use MetaContext to terminate instead of meta_quit()

This is done by keeping around a pointer to MetaContext as
"client_pointer" (which is practically the same as "user_pointer"
elsewhere), as well as creating a `MetaIceConnection` wrapper for ICE
connections.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/meta-context-main.c

@@ -460,7 +460,8 @@ meta_context_main_notify_ready (MetaContext *context)
 
   if (!context_main->options.sm.disable)
     {
-      meta_session_init (context_main->options.sm.client_id,
+      meta_session_init (context,
+                         context_main->options.sm.client_id,
                          context_main->options.sm.save_file);
     }
   g_clear_pointer (&context_main->options.sm.client_id, g_free);