One xcb module, one header file: <xcb/xcb.h> should to be
included directly, instead of relying on other modules dragging
it in; on the other hand, it's okay to include just the top
level <xcb/xcb_util.h> instead of the three separate sub-headers.
[rk@rejii.com: skip xcb_util.h since it breaks xcb-util < 0.3.8]
closes #142