[xmonad] darcs patch: add configurable X Event handling

[Devin Mullins]

> At this point, I wonder whether we should follow the layouts more closely
> and make the MessageHooks be data rather than functions, so that they can
> store state that persists across mod-q.  So we'd have something like:
*That's* specifically what I don't want. The whole point of eventHook
(or messageHook) is that it avoids the Read/Show requirement, so users
can specify custom (in this case) urgency hooks without a load of
class/instance boilerplate. Otherwise, I'd just leave it as a
LayoutModifier, as it is now.

I'm intrigued with the state idea. What did you have in mind?