WebRTC.MediaHandler/MediaStreamManager: support multiple MediaStreams

MediaHandler:
  #addStream -> #addStreams: takes (Array of) MediaStream
  #render: renders only first local/remote stream (see #76)

MediaStreamManager:
  .render:
    * takes (Array of) MediaStream
    * renders only the first (see #76)

  #acquire:
    * returns a Promise of Array of MediaStream
    * mediaHint.stream is (Array of) MediaStream

  #release: takes (Array of) MediaStream
WebRTC: isSupported doesn't require getUserMedia

This allows the default MediaHandler to be used without capturing media.