Add funcionality to Roll Metacity Windows Down

I think it would be very useful to choose the direction of the roll effect of metacity windows.

Since I use only one gnome panel on the top of the screen it would be very nice to have some windows (buddy list, chat window, gwibber and such) acting like pop-up panels that could be rolled down and snapped to the bottom of the screen.

Once rolled, when one of this windows got focused they would unroll until unfocused or explicetally unrolled.