imageinfo.identify(): improve description about exceptions
Metadata.append_image(): remove useless assert()
Replace save_this_image_to_tags() with Metadata.images_to_be_saved_to_tags

It simplifies code, and it fixes the code, using is_front_image() instead of is_front.
Initialize CoverArtImage.is_front to None, and don't specify default type

- is_front is used when one set "front" explicitly (CAA does it)
- types is now empty by default
- rely on is_front_image() to handle various cases
- if support_types is False, is_front_image() will default to True
CoverArtImage.maintype defaults to 'front' if is_front is set or no type set
Improve __repr__() outputs
Improve tests for cover art types

- add tests according to save_only_front_images_to_tags
- some formats cannot save more than one image
- some formats have no type
Add missing import
Use is_front_image() instead of is_front
types_and_front() -> types_from_id3(), setting is_front isn't require anymore