wayland: Add support for xdg-shell stable, take 2
This is a notable rework of the wayland/xdg-shell MR !60 (closed) by Scott.
In a Tl;Dr:
- core wl_interfaces like wl_compositor_interface are now embedded (thanks for the idea Pekka)
- there is no need to hack/patch the protocol files
- meson + cmake autodetect and build properly
Note: the full screen/maximise config events are not handled. From current testing things just work(tm) as-is. I don't mind following up as we get a use case for them.
- keep the old wl_shell interface and thus preserving the ABI
- add new getters API for the xdg-shell based implementation