Rework central image loader to better handle PDI format

When loading an image, certain actions are relevant for flat images
(JPEGs, etc), but not for PDIs (which have already had such processing
done to them).  The master load function has been reordered to reflect
this.

Also, Undo/Redo no longer overwrites pdImage properties unless they are
relevant to the Undo/Redo stack.  (For example, image size is something
that can be undone/redone, but image location on disk is not.)