Do not delete media when their parent folder gets removed
In order to preserve known information, we need to switch from cascading the deletion of a folder (when removed from the indexed folder list) to converting any media that would be removed into an external media.
This would allow to:
- Keep the media in any playlists without ugly kludges
- Preserve the user provided information
When converting a media from "internal" to "external", we still need to remove it from any album/artist/show it was linked to.