[Xcb] doxygen doc of the API

[Vincent Torri]

 Here is a small example of the documentation:

http://www.iecn.u-nancy.fr/~torri/files/xcb/xcb_api.tar.bz2

 What can be changed:

 * the Doxyfile options
 * the css style sheet

 i also would like to add some presentation on the main page. For example:

 * general presentation (from the wiki)
 * a small example of code
 * something else ?

 we can add some todo items, if some functions need to.

 comments ?

 Vincent