[xmonad] GridSelect patch

The attached patch modifies XMonad.Actions.GridSelect to support running the 'gsmenu' program (which is on Hackage) instead of the built-in stuff. The external API is unmodified (and I had to make a few ugly-ish hacks to keep it that way) and the default settings are completely unchanged. When using the external gsmenu, all options (except for key bindings) are still accepted. -- \ Troels /\ Henriksen