GNOME Mutter Merges XDG Session Management Wayland Protocol

As a very last minute change ahead of tagging GNOME Mutter 47, merged this morning to Mutter is support for the XDG session management Wayland protocol. This protocol is useful for letting clients request support from the compositor for saving the window state for use on future executions. However, it's currently disabled by default and won't be entirely baked until GNOME 48...