refactoring find_pathes for images.
 1. <basedir>/<chapid>-<id>.<ext>
 2. <basedir>/<builder>/<chapid>-<id>.<ext>
 3. <basedir>/<chapid>/<chapid>-<id>.<ext>
 4. <basedir>/<chapid>/<id>.<ext>
 5. <basedir>/<builder>/<chapid>/<id>.<ext>
 6. <basedir>/<builder>/<id>.<ext>
 7. <basedir>/<id>.<ext>
drop <basedir>/<chapid>/<chapid>-<id>.<ext>
improve the image file finder.
Now review-compile finds a image file from:
1. <imgdir>/<builder>/<chapid>/<id>.<ext>
2. <imgdir>/<builder>/<chapid>-<id>.<ext>
3. <imgdir>/<builder>/<id>.<ext>
4. <imgdir>/<chapid>/<id>.<ext>
5. <imgdir>/<chapid>-<id>.<ext>
6. <imgdir>/<id>.<ext>