[Xcb] api documentation with doxygen

[Jamey Sharp]

Hey Vincent,

Your Doxygen patch looks fine to me. If you're waiting for my approval
to start committing that sort of thing, you have it. :-)

I want to point out that the XCB protocol stubs can't be easily
documented this way, though. I want to have protocol documentation start
being folded into the XML descriptions, and then we'll be able to
generate byproducts like Doxygen comments from there. (Not to mention
reproducing something much like the usual protocol specifications.)

Any comments on what documentation tags we might use, and where? I think
re-using an existing XML documentation language would be a big feature.
DocBook comes to mind, though if there's a standard XML form for Doxygen
maybe that would be a good choice.

--Jamey