MediaGroup media count should only reflect present media
Otherwise nbVideo/nbAudio/nbUnknown will return values that differ from the media listing query.
This also implies that we need a media count that doesn't take into account the presence of the media, which will be used for automatically delete empty groups.
- Add a new field to count media regardless of their type and presence
- Update the nb_video/audio/unknown trigger to ignore updates on missing media
- Add a new trigger to increment/decrement the nb_video/audio/unknown when media presence changes