(NEW FEATURES): Completed work on this item:

(NEW FEATURES): Completed work on this item:
Rework how fringe bitmaps are defined and used.
Currently, bitmap usage and bitmap appearence are "mixed-up" in a
one-level representation.  It would be cleaner to split the
representation into a two-level model where first level maps
bitmap usage to a bitmap name, and second level maps bitmap name to
a bitmap appearence.