[xmonad] Some added 'all windows on current screen' functionality

Hey List, Here's a bit of code i've found useful in my config. You can think of it as the 'get your *** out of work quickly card', as well as the 'oh ****, hide all the open porn!' button. Admittedly, i'm not sure it belongs in SinkAll, but that's where i shamelessly stole the code from. It also didn't seem like enough to actually spin out a new module just because. I'll let you all decide where it should go. Personally, i really don't care as much, just so long the functionality is there. :) Cheers, Yaakov