libkate: dependency fixes

Does anyone actually use this package heh? I’m amazed nobody has noticed it’s been really quite broken for a while. 

It hasn’t been building the ogg libraries because it finds that dependency through pkg-config which we hadn’t specified as a dependency, and the only other thing that hooks into libkate looks for those ogg libraries and fails to find those consequently. 

This also adds optional dependencies for documentation, examples and the GUI for libkate. Libogg also isn’t really a recommended dependency, if we’re going with libpng being mandatory libogg should be as well.

Added a mirror and a simple test as well.