[xmonad] darcs patch: Applicative instance for X; new startupHook

[Don Stewart]

byorgey:
>    Hi all,
> 
>    Here are two patches to the core, one to add an Applicative instance for
>    X, and another which adds a startupHook :: X () to the config.  I wrote
>    them to support updates to PerWorkspace (patch to follow shortly), but I
>    think they're both useful in and of themselves, especially the
>    startupHook. 

Can we derive the Applicative instance? (Are applicatives in mtl yet?)