compositor: overhaul Wayland backend

This simplifies the Wayland backend, fixes the build configuration for it, ports it to xdg-shell stable, and reworks some false assumptions from the original port. Note that this does not add the Wayland backend to cmake.

That this also raises the priority of the Wayland backend under WINDOW_AUTO above the xcb backend, otherwise the Wayland backend is never used on compositors which support Xwayland (namely all of them).