plugin XEP-0167: handle conferences: - SDP can now be answered by component instead of frontend. This is useful for A/V conferences component to handle A/V call jingle sessions. - new `call_update` and method, and `content-add` action preparation. This is not yet used by A/V conference, but it's a preparation for a potential future use. - Add NS_AV_CONFERENCES to features as required by the newly proposed A/V Conferences protoXEP. rel 447