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 (using thumbnail)
- PDFs will only downloaded if option to save cover to filesystem is set (caa types filtering still applies)

![capture du 2014-05-28 17 26 53](https://cloud.githubusercontent.com/assets/151042/3106583/2ce215ba-e67d-11e3-8c30-47009baa2b90.png)

http://tickets.musicbrainz.org/browse/PICARD-572