Support loading libxcb.so and friends through dlopen

[maroider]

This would allow for building binaries which can run regardless of whether or not the user has libxcb installed. x11-rs already does this by providing the x11-dl crate (after it was requested). This is particularly useful for applications which want to work under both X11 and Wayland.

[rtbo]

That feature will be much easier from v1.0 as there is no more linking to the extension libraries (only libxcb.so is linked to).
I will target this for v1.1.

[Be-ing]

The dlib crate is great for this. I recently helped integrate it in the Rust JACK bindings.

[Be-ing]

It looks like it would be tricky to use dlib for this due to this crate's custom code generation. It might be better to implement this using the custom code generator.