[xmonad] Workscreen extension

Hi, I'm glad to send you a patch to introduce 'workscreen' concept. A workscreen permits to display a set of workspaces on several screens. In xinerama mode, when a workscreen is viewed, workspaces associated to all screens are visible. The first workspace of a workscreen is displayed on first screen, second on second screen, etc. Workspace position can be easily changed. If the current workscreen is called again, workspaces are shifted. This also permits to see all workspaces of a workscreen even if just one screen is present, and to move windows from workspace to workscreen. I use XMonad since 3 years, and I want to thank you for this WM. Hoping that you will be interested by this extension, Yours sincerely, Dal