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