Bugfix: Properly integrate libxcb’s event loop into libev.

Bugfix: Properly integrate libxcb’s event loop into libev.

Fixes a race condition with GIMP (where it configured its window,
sent the map request and waited for the window to get mapped, but
i3 didn’t get the event until another one was sent (key binding for
example)).

The new solution is much better as it properly hands off all
the work to xcb_poll_for_event.

Inspired by awesome, which uses the same mechanism. Thanks.