webrtc: Initial support for stream addition/removal (renegotiation)
Limitations:
- No transport changes at all (ICE, DTLS)
- Codec changes are untested and probably don't work
- Stream removal doesn't remove transports (i.e. non-bundled transports will stay around until webrtcbin is shutdown)
- Unified Plan SDP only. No Plan-B support.
- No concurrent negotiations
Edited by Sebastian Dröge