[xmonad] darcs patch: floating layout with placement

Hi Attached is a patch that introduces an XMonad.Layout.PlacedFloat module, similar to SimplestFloat but with windows automatically placed using a configurable policy. The patch also refactors X.H.Place and moves the placement algorithms to a new X.A.Place module, but all of this should be pretty much backwards-compatible except for the disappearance of purePlaceWindow. Cheers, Quentin