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.

/cc @pq @daniels @oreaus @dbaker

Edit:

TODO:

• keep the old wl_shell interface and thus preserving the ABI
• add new getters API for the xdg-shell based implementation