Big rework of image.c and the image cache system.

Added a lot of comments, simplified call graphs.
Added full (not second-class) support for images coming from named sources (files, icon themes).
RrImage holds an RrImageSet.  RrImageSet holds a bunch of RrImagePic, which are different sizes of a logical image.
RrImageSet objects can be merged if it is discovered they (will) share an RrImagePic.  The RrImage objects are updated to use the new merged RrImageSet.