Media: Rework Type management

We need to allow an external application to provide the media library with the Media type.

This will come handy when we need to know a media type as early as possible when trying to restore the saved states as we don't want to do this for audio files (or at least not podcast, but those aren't supported by the media library for now)

This implies to:

• remove External and Stream from the media types.
• Add a setType method to IMedia
• Allow media without tracks to be added to the media library, with the type Unknown
• Trigger a refresh when a media type changes from Unknown to Audio/Video