VLCMetadataService: Don't unregister events manually
VLCMetadataService: Don't unregister events manually

We'll soon start monitoring more than a single event, so let's prepare
the code for that by not having to manually unregister things and store
multiple RegisteredEvent instances

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary