Fix regression, save only ONE front image when "save_only_front_images_to_tags" is set

This regression was introduced during code rewrite, option name is misleading.

See https://github.com/musicbrainz/picard/pull/116
See PICARD-350 (which wasn't closed)
Only download one front image if no more is needed according to options
Merge pull request #333 from zas/fix_front_image_only

Fix regression, save only ONE front image when "save_only_front_images_t...