Partial fix for #40
Partial fix for #40

Improvements:
 - clicking on the root will change focus to that screen
 - moving the mouse from a window on a screen to an empty screen changes focus
   to that screen
The only remaining issue is that moving the mouse between two empty screens
does not change focus.  In order to solve this, we'd have to select motion events
on the root window, which is potentially expensive.

pjones/xmonad-test,xmonad/xmonad,ohlus/xmonad,ericssmith/xmonad