Wayland XDG Shell interface

Adds a new directory with the protocol file. I hope @wm4 doesn't kill me.

**NOT (FULLY) RELATED TO THIS PULL REQUEST**:
I created the folder `video/out/wayland/` because I like to split up the `wayland_common.c` and maybe also `vo_wayland.c` stuff up and put them into `video/out/wayland/`. The source files are getting bigger and It gets harder to find stuff. Also in the future with `scaler.xml` and `dmabuf.xml` it will be possible to have different scalers and buffer providers which would be better separated into different files.