Add image description to debug message if available
Remove useless import
Add types_and_front() helper function

- it converts id3 type to CAA-like type list
- it returns a is_front boolean, emulating CAA flag
Improve handling of multiple image types and is_front flag

- convert type-as-a-string to list of types
- display concatenated list of types in messages (ie. back,spine)
- pass is_front flag everywhere needed
- pass list of types instead of single string everywhere needed
- introduce maintype() method to get the first type if more than one is set
- in formats/*, use types_and_front() at read time to get properly formatted list of types (only one element for now, since no format seems to support multiple types)
- use Image.maintype() at write time (since no format supports multiple types for now)