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.

(full debug output will be disabled for the next release)

This should fix a race condition where the Mod-key was not released
in time when starting i3lock using a key combination.

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)

When suspending your system to RAM, a race-condition can occur which
causes many enter keypresses to be sent. These effectively delay
your login by 3 seconds times 50 times (or so), so we just skip empty
passwords. If your password is empty, you don’t need to lock your
screen anyway.