Adding in API to allow for DVR information used to drive a player UI

Added new class called videoElementExt with and API to help with DVR window. We could add other video element API to this as well in the future. Added metric to hold the information for DVR streams. Wanted the API to be a singleton and the metric VO to just hold the values for the API to use.
I agree we should not have it in both places.  If we agree to move the time duration API to media player then I can either forward the info from the DashManifestExtensions at the media player level or just remove the call completely. I do know from experience this is something that all players will want to know so they can decide if they should add a seek bar or not.   So it should be easy to fine and an obvious API name