And use the loudness measurement as a first use case.

The main difference between metadata listeners the player listeners are:

 - The information returned by metadata events is mainly useful for the UI, it
   should not be used to control the player.

 - It's not possible to call or lock the player from metadata events

 - Registering a metadata could cost some CPU cycle since it may spawn a
   measurement filter to get the requested metadata. Such cost should be
   explained in the comment of vlc_player_metadata_option enum.

Some player events could be moved to metadata events, like the statistics one.