Set PeerConnection streams API as an extension

Reading the specification, I have seen that the audio & video stream
has a "privileged" status being  defined their API directly on the
PeerConnection one, while DataChannel, DMTF, statistics and identity
APIs are specified as a "PeerConnection extension". I propose for
homogeneity to set the PeerConnection specification to their basic
functionality removing and setting in an independent section the next
methods and their related issues:

    sequence<MediaStream> getLocalStreams ();
    sequence<MediaStream> getRemoteStreams ();
    MediaStream?          getStreamById (DOMString streamId);
    void                  addStream (MediaStream stream, optional
MediaConstraints constraints);
    void                  removeStream (MediaStream stream);

                attribute EventHandler          onaddstream;
                attribute EventHandler          onremovestream;

They could be included and defined in the section "9. Media Stream API
Extensions for Network Use".


--
"Si quieres viajar alrededor del mundo y ser invitado a hablar en un
monton de sitios diferentes, simplemente escribe un sistema operativo
Unix."
– Linus Tordvals, creador del sistema operativo Linux