Better handling of metadata image types

Keep full image type from CAA/MB (ie. front,back,spine)
When option is set to save image to local files, name is build after complete type, ie. front-back-spine.jpg
Introduce main cover concept, since front type wasn't clear enough.
Main cover is:
- first image with front type
- first image if no front found
- image set by user by drag'n'drop on cover art box
- there's only one main cover
- it is used in place of "front", even though it is often the same,
  especially when only one image is found
- it is the image displayed by picard for the album

To ease things a new class MetadataImage was added and is now used for Metadata.images elements.
Naming of image files (when saved to local) was modified, and now use a callback
function to generate it (mostly because it depends on main cover and options set), it also helps to clarify code

Info box was improved too, it now displays image type and comment (as well as filename in tooltip), main cover is printed in bold.
![shot](https://f.cloud.github.com/assets/151042/59574/e21dd6ca-5bd3-11e2-8e12-dec5c83394bc.jpg)

Also it is a step to a proper fix of issue PICARD 350 (http://tickets.musicbrainz.org/browse/PICARD-350), reporter
is in fact requesting for a way to save _only_ the main cover art image