Feature request: add a callback when a media is marked as external media
Context: on Android, some of our thumbs are given by the system and stored in the FS. So the medialibrary mechanism purging the outdated thumbnails cannot be applied there.
When a media is deleted (from the FS or from the ML), the app gets an
onMediaDeleted callback and is able to purge the thumbnail.
As the thumbnail can be re-calculated on-the-fly by the system if needed, there is no need to keep the thumbnail file when a media is not indexed anymore (when it become external in that case). That's why we need a callback in that case.