Fix new book thumbnails on Linux (BL-505)

Won't this actually cause the cursor to turn into a wait cursor if the user hovers over this before it gets updated?

Anyway, why use a hack? _buttonsNeedingSlowUpdate is a private variable, and only two bits of code are involved in recording and using the information about whether the thumbnail needs updating. How about just changing it to a queue of Tuple<Button, boolean> or make a little private class that stores a button and an ThumbnailNeedsUpdate field and queue those?