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
- 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)