store cached as scaled thumbnails

[brainchild0]

Currently, Bookworm appears to cache thumbnails on disk in their original form. If the files are large, then the effect is to consume an accumulated amount of storage much heavier than needed. Since the requirement for the application to display rapidly large numbers of covers is limited to thumbnail-size representations, perhaps images can be reduced in size (and even compression quality) to some upper limited that is required for library browsing.