player: add a metadata listener API
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.