[xmonad] darcs patches: Support for extensible state in contrib modules

These patches add support for extensible state in contrib modules to remove the need for having the user create IORefs for that. This somewhat simplifies the interface of A.SpawnOn and H.DynamicHooks and removes the use of unsafePerformIO in H.UrgencyHooks. This will break configs, since one no longer has to pass around IORefs, but since this makes those modules somewhat easier to use, I think that's tolerable. Those patches also fix issue 78.