Wayland backend xdg
A few notes:
- I made a new list for protos to only generate some as client headers, we might want to always generate them and simplify that.
- as described in 2nd commit message, failing to create any display does not fail the backend, it probably should.
- I didn't find how to set a class, closest I could find was app_id. Should I? (there is a comment in the code)
Rest looks like it works ok.