[xmonad] darcs patch: ManageHelpers, reduce duplicated code in predicates

This patch adds a helper function isInProperty to reduce the duplicated code in isFullscreen and isDialog of ManageHelpers. Next step: making ManageDocks use this function or similar to avoid more duplication. Next next step: pushing this helpers (or in another way) into core, I think that providing a common way of doing certain things helps avoiding code duplication in Contrib. byebye! P.S: (this is my first code patch, so be gentle :) )