[xmonad] New layout and patch for XMonad.Actions.Warp

The first patch adds a new module XMonad.Layout.GridVariants. It provides a version of the Grid layout that allows the desired aspect ratio of the grid windows to be specified. It also provides a TallGrid layout whose master area is a grid with a specified number of rows and columns and whose slave area is a grid with windows of specified aspect ratio. I believe that this isn't easy to achieve using layout combinators. The second patch fixes the documentation of XMonad.Actions.Warp.banish and implements a new function banishScreen that does what the old documentation of banish claimed banish does. Cheers, Norbert