Improve handling of PDF cover art from CAA

- detect PDFs in imageinfo
- exclude PDFs from images to be written to tags
- display info about PDF in info dialog (need more work)
Download pdf cover art only if needed, download main file, not thumbnails.

This patch solves 2 issues:
- pdf files only need to be saved if "save_images_to_files" option is set, PDFs are never embedded in tags
- ignore "caa_image_size" option which causes thumbnail of the pdf to be downloaded instead of the main file
CaaCoverArtImage: specify constructor, accept is_front
Move _queue_from_caa() code to _caa_json_downloaded(), simplify
Display thumbnail image for PDFs in info dialog

It requires few changes in CaaCoverArtImage, basically one property (is_thumbnail) to indicate
the image is a thumbnail so it isn't added to metadata.
And a thumbnail property to link main image to its thumbnail.
CaaCoverArtImage: describe `thumbnail` and `is_thumbnail`
Introduce CaaThumbnailCoverArtImage and properties to handle thumbnails

- can_be_saved_to_tags
PDFs cannot be saved to tags as today

- can_be_saved_to_disk
thumbnails shouldn't saved to disk (in audio files directory), they are only used for display

- can_be_saved_to_metadata
thumbnails aren't saved to audio files metadata
Eventually raise CoverArtImageIOError when getting CoverArtImage.data
images_to_be_saved_to_tags(): use can_be_saved_to_tags property
Merge pull request #332 from zas/cover_art_pdf

Improve handling of PDF cover art from CAA