Fix spaces for gir scanner
Refactor event and reply types to includes

This is a major refactoring that moves all event boxed types to
i3ipc-event-types.[ch] and all reply boxed types to
i3ipc-reply-types.[ch]. This should make the code more readable and the
sections of the documentation look nicer.
Add gtk-doc support

Installation now requires gtk-doc. Reference documentation builds in
doc/reference. The actual documentation is a work in progress.