[xmonad] darcs patch: Column layout

Hello xmonad world. This module defines layot named Column. It places all windows in one column. Windows heights are calculated from equation: H1/H2 = H2/H3 = ... = q, where `q' is given (thus, windows heights are members of geometric progression). With Shrink/Expand messages one can change the `q' value. I use this layout for Gimp's image windows - such, with 2 images and (Column 2.0), first image window occupies 2/3 and second - 1/3 of screen height. WBR, Portnov.