Move wayland-cursor and wayland-client documentation from implementation to header

The documentation for wayland-cursor wasn’t even built, so for users the only way to access it was to download this repository and read the .c file, obviously not something users should do when they already have the .h installed in /usr/include for all to read.

This MR also does the same treatment for wayland-client, configures doxygen to generate documentation for wayland-cursor, and document its structs.

Fixes #116 (closed).