waylandsink: Support xdg_shell stable

waylandsink currently only supports the ancient wl_shell interface for window management, which was never intended to be used for anything real.

wl_shell is long since deprecated in favour of xdg_shell, which has now been made stable and implemented across all of Mutter, QtCompositor, Weston, wlroots, and more: you can see the status of stable xdg_shell adoption also.

This involves using xdg_wm_base as the replacement for wl_shell, and a combination of xdg_surface and xdg_toplevel as a replacement for wl_shell_surface. Notably, xdg_shell allows far better negotiation of window size and state such as fullscreen, maximised, tiled, etc.