This patch ensures that the focus (or in fact the whose windowset)
does not change as a result of a banishScreen. The way this is implemented
will become problematic if xmonad ever goes multithreaded.
warp 1 1 has a comment claiming that this moves the cursor to the lower *left*, but if you look at the warpToWindow haddock, it says that 1 1 is actually lower *right* - as indeed it proved to do. This was annoying as it led me astray for a minute or so.
