Things we're currently missing from the wlc API

[SnirkImmington]

Here's a list of functions from the wlc API that we have not implemented yet:

:x: wlc_exec - Rust already supplies this.

• [x] Functions with wlc_resource or wl_resource - Some of these require partial bindings to wayland, and wlc_resource seems ill-defined/we don't get it.
• [x] wlc_handle_{get,set}_userdata - Uses a void* and is under-documented. We've also split wlc_handle up and would need to take that into account.

Optional bindings for wayland features - using an existing Rust crate as an optional dependency for wayland bindings, add the wayland features wlc exports. Put this under a feature.

• [x] wlc_wayland.h
• [ ] wlc-render.h

If you have a pull request for any of these features you'd like to submit, we would be very appreciative.

Otherwise, if there's something that we've left out because we didn't RTFM and you have a manual, please point us to it and we'll add it to our to do list.

[SnirkImmington]

Added wlc_handle_get/set_user_data

[SnirkImmington]

Add optional dependency for wayland stuff

[Timidger]

This will be fixed #62 is merged in.