Add the API and IPC required to pass the surface placement requests
needed to support gdk_window_move_to_rect() to the window manager. It
does not include the window manager logic to service the request.
(There's an associated window management implementation proposed for
MirAL: lp:~alan-griffiths/miral/connect-gdk_window_move_to_rect-inspired-placement-logic-to-Mir-0.25-API/+merge/304481)