I've added support for setting the fullscreen state of a window, as well as toggling it through a keypress. I now need to set fullscreen when client's request it.
Added fullscreen Doxygen comments.
Howm can now handle clients telling it when they want to be fullscreen or not.

I also fixed a subtle bug that meant that fullscreen windows weren't always focused.
I neatened up the client message code and made it easier to understand. I also added Doxygen comments for it.