Handle read only playlists
Until we properly implement import/export for all supported playlist format, we need to let the application know if a playlist comes from a file on disk.
If so, then the playlist should be considered as read only, otherwise the version in database would diverge from the version on disk, and any modification to the file would discard the changes in database.
Attempts to modify a read only playlist should result in an assertion failure in debug builds, and just return
false in release builds