Rework thumbnailing API

As a follow up from #23 (closed), #88 (closed) and #82 (closed), we need some changes to the thumbnailing API.

• Add a thumbnail size type
• Remove Thumbnail.origin field
• Add per-entity thumbnail linking tables
• Remove entities thumbnail_id fields
• Use the media progress meta to compute the thumbnail, default to 15% (less than 30% to avoid spoilers :) )
• Provide the position to generateThumbnail
• Accept thumbnail generation request on a media which already has a thumbnail (and ensure this is properly documented)
• Move thumbnail generations request from IMediaLibrary to IMedia
• Add thumbnail size type, desired width & height parameters to IMedia::generateThumbnail
• Add a thumbnail size type parameter to IMedia::isThumbnailGenerated
• Ensure the resulting thumbnail respects the aspect ratio & is cropped to fit if necessary
• Index linking tables thumbnail_id field
• Automatically remove unused thumbnails (ie. a thumbnail_id which is never referenced from any linking table)

Cropping will most likely require a VLC core API change, since for now only scaling by respecting the aspect ratio appears to be supported by picture_Export