When "save_only_front_images_to_tags" is set, save only 1 front image.

Introduce helpers to clarify and fix code:
- is_front_image()
- save_this_image_to_tags()
- image_type_from_id3_num()
- image_type_as_id3_num()

Fixes:
- honor "save_only_front_images_to_tags" option for apev2 format too
- save only caa front image to tags when "save_only_front_images_to_tags" is set
  and image source is CAA, use 'front' flag from json.
  Other sources should have only 1 front image anyways.
Merge pull request #116 from zas/front_image

When "save_only_front_images_to_tags" is set, save only 1 front image.