Speed up ThumbsWindow with multiprocessing

[Peter Levi]

We can use Python's multiprocessing module to offload thumb-building to separate processes.
This would make the ThumbsWindow appear smoother and faster. This may very well also fix the seldom crashes which happen while interacting with it while it is being built. Also, it may allow us to use a higher image count limit for it (100 seems low).

[Peter Levi (peterlevi)]

This does not seem super important now that we reduced ThumbWindow sleep calls - the thumbs are rendering quite fast. And with all of multiprocess-related communication overhead, need to encode and decode the images, it may turn out here is little performance benefit.

Still worth testing, but not too urgent.