Migrate to libxcb if possible

[iphydf]

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.