Rework thumbnails

This reworks the thumbnail handling, which mostly comes down to:

• Extracting the cover images from the medialibrary instead of relying on VLC
• Sharing the covers as much as possible between entities

What is yet to be solved:

• How to handle attached artwork? Apparently there's no way to disable local art fetching when using libvlc. So if we reject everything from VLC, we lose attached artworks, if don't, we would end up duplicating the album cover for each tracks. Sadly we can't rely on the artwork MRL being "attachment://" when fetching it from the media, as it might or might not be extracted already after a preparsing (depending on which module was used, AFAICS)

Postponing a finer approach for VLC 4.0, where we can have a proper API to do the extraction. Mobile ports will always copy the thumbnails. When possible, the thumbnail will simply be a hardlink.

• Use embedded artwork as a album cover

Postponed until we have a better embedded art management

• Most likely fix win32 build breakage