[xmonad] New module: XMonad.Layout.Fullscreen

The attached darcs patch to XMonadContrib adds a new module implementing a (IMHO) much better way of handling fullscreen windows than the existing fullscreenEventHook from EwmhDesktops. Instead of simply fullFloating the window, this module sends a layout message so that the layout can itself determine how to handle the window. Three proof-of-concept layout modifiers are provided, that illustrate the flexibility of this approach: Two varietes to add fullscreen handlig to the layout, and one hackish modifier that adds fullscreen handling to the floating layer. All of these modifiers also have variants that allow you to specify which part of the screen the fullscreened window should fill. I hope you will find this useful.