[Xmonad] darcs patch: use a global IORef to keep list of urgent windows

[Don Stewart]

me:
> Somebody asked for the ability to display which workspaces had urgent windows in them in the logHook.
> Well, here it is. (Well, the core state-tracking functionality.) Please provide input on:
>  - If it's bug-free (no space-leaks, safe-ish unsafePerformIO, etc.)
>  - If it's useful (whoever you were that asked for it)
> 
> Sat Oct 27 02:48:10 EDT 2007  Devin Mullins <me at twifkak.com>
>   * use a global IORef to keep list of urgent windows
> 

Applied.

-- Don