optimization: don't redraw for every time windowsChanged is called. in
the case of moving a window when desktop previews are on, moving a
window across the screen at a reasonable pace used to result in over 100
repaints. now it only does ~15. given that the repaints aren't exactly
trivial for these things, this is probably a good thing.

for maksim. and njaard, just because he's always bitching at me about
kicker.