Rework thumbnail inheritance
We currently have a rather limited thumbnail inheritance support.
We make huge assumptions based on the thumbnail origin, which is not enough. We need an extra field in DB to know if a thumbnail is shared among various entities, and only update the actual thumbnail when it's not.
In addition, we will need to explicitly provide the origin of the new thumbnail (the one conveyed in the
Thumbnail object may not be enough, for instance when assigning a thumbnail to the artist, the thumbnail origin will almost always be
Media, while we need to know if the thumbnail comes from an album artist or an artist in order to address #106 (closed))