Before this commit, the background color (white by default) was visible for
about 100ms until the image was drawn. This flickering is now eliminated.

Also, we don’t need to handle Expose-events anymore, as X11 will use the
window’s background pixmap automatically.

As explained in Jamey’s post on the XCB mailing list, the event
loop can/should be written in a more simple way than it was before
in xcb-event. See:
<AANLkTinhIM320wUjSeHkBTVk4ysBuiSZkvfs8pMLVykO@mail.gmail.com>
or
http://lists.freedesktop.org/archives/xcb/2010-July/006292.html

Changes:
- Uses XCB instead of Xlib (like i3), thus cleaner/faster code
- Uses cairo to load PNG images (*much* faster than XPM)