[Xmonad] darcs patch: only display any given window once.

Hi all, This change is independent of the change to delete. Together they make sticky windows (via the CopyWindow contrib module) work nicely with Xinerama. Without this change, a gap will be visible on one or more screens when a sticky window is present in workspaces present on multiple screens, which is not the end of the world, but is a bit ugly. With this change, each window is displayed on only one screen. Note that this is just an amend-recorded version of my earlier patch, fixed to work with the modified API. David Tue Jul 24 07:13:10 PDT 2007 David Roundy * only display any given window once. This change goes along with the sticky window work. It makes xmonad display each window once and only once, with preference given to the focussed screen. It has no effect when there are no duplicate windows, except to make things less efficient. We could do better using Data.Set (or Data.Map) to store the set of windows that are visible.