Use embedded cover in MKV (and others) video files
Only MP4 embedded covers were supported, using TagLib. This MR abandon TagLib (removing a few "deprecated" warnings in the process, as well as TagLib dependency) in favor of ffmpeg itself, which supports MP4, MKV and others' embedded covers.
We also rely on Matroska specification's attachment names to select the best cover (landscape preferred) if several are available (https://matroska.org/technical/attachments.html).
Edited by Amaury Bouchra Pilet