Support for reading new Vorbis cover art

http://wiki.xiph.org/index.php/VorbisComment#Cover_art

https://bugzilla.gnome.org/show_bug.cgi?id=689420
Support for writing new Vorbis cover art

http://wiki.xiph.org/index.php/VorbisComment#Cover_art

https://bugzilla.gnome.org/show_bug.cgi?id=689420
Mark Vorbis tags with deprecated images as changed

Ensure that old COVERART fields containing cover art in Vorbis tags are
migrated to the new METADATA_BLOCK_PICTURE by marking the tag as
modified, so that a save will be suggested (in the new format).

https://bugzilla.gnome.org/show_bug.cgi?id=689420