Add standalone wallpaper changer..

It seems to work fine for me here except for the fact that I get 'failed to create drawable'
messages, which doesn't sound good.  I wonder if the bg object is expiring before
the X server gets around to actually changing the pixmap, but I get the warning
even if I add time.sleep(5) or something.

Anyhow, I fixed up backgroundManager.js to only call the script on changes.