1) Move namespaceURL out of Metadata, it now becomes the key whenever Metadata is the value. The access model from MediaElement is now patterned off the access model for traits.
2) Add namespaceURL to MediaElementEvent, to enable identifying which Metadata the event corresponds to.
3) Add get metadataNamespaceURLs to MediaElement, to enable iteration over metadata.
4) Have TimelineMetadataEvent extend MetadataEvent.