[xmonad] darcs patch: hide the implementation type in "EwmhDesktopsLayout"

Hi, this is what I hope a slight improvement over Malebria’s patch – thx by the way. Greetings, Joachim Tue Jun 10 21:29:18 CEST 2008 Joachim Breitner * hide the implementation type in "EwmhDesktopsLayout" Instead of exporting the EwmhDesktopsHook type, which is used internally, export a type alias for (HandleEvent EwmhDesktopsHook), so users don't have to update their xmonad.hs if the implementation changes. It could also be wrapped in a newtype, instead of a "type", but that would require manual instances of the various classes, as ghc can automatically derive the LayoutClass instance.