! Let's replace those sort_up.png and sort_down.pngs more generally with...

... a single image sprite. For those worrying about backwards compatibility, there already wasn't because the old images were GIFs, these are PNGs, so old themes would already be a bit iffy anyway. But now it's styleable from CSS - and that also means it's variant-safe as well should someone wish to do something with that.

Signed-off-by: Peter Spicer <sleeping@myperch.org>