Migrate to libxcb if possible

Advantages:
* Proven thread-safety.
* Much easier to build.
* More portable.
* Asynchronous API, so no need to do threading ourselves (less room for breaking thread safety).
* More stuff that's less relevant to us, but still: https://xcb.freedesktop.org/Features/

I'm struggling a bit with xlib, so I'd rather ignore xlib and move to xcb.