Convert internal media to external when removing their parent folder

For now when an entrypoint gets removed, we delete all the media it was containing.

Since this should also remove the media from the playlist it belonged, we have a hack/workaround in in the playlist code that will replace the media_id by NULL, and tries to recover from a NULL media_id during the next access to the playlist. As it turns out, this is a major impediment for implementing #292 (closed) since we don't know if the media was removable or not after its been deleted, or it requires us to have 2 ways of updating the media counts.

TL;DR: Let's kill the playlist media_id hack and implement a proper solution.

When an entry point gets deleted, we should keep all the media it contained, and convert them to external media. If the folder comes back, we can convert it back to internal.

We probably should delete any information that was extracted from the media (ie. albumtrack, video/audio tracks, artist membership, ...) so that the media behaves just like a regular external media.