Enable renegotation

[farao]

Roughly following https://blog.mozilla.org/webrtc/perfect-negotiation-in-webrtc/
but using @negotiationInProcess instead of @peerConnection.signalingState to
make the process a bit more readable/understandable.

Disadvantage: Rollbacks are quite new (half a year in chrome, bit more in Firefox).
But: Races should be rare and all other solutions would require a protocol change
and would make the process slower.

[farao]

Needs testing :-)

[farao]

Three example scenarios which should hopefully be supported by enabling renegotiation:

1. Screen sharing (adding a screen video track on the fly)
2. Starting audio only and deciding to want to switch on the camera during the call
3. Changing networks, e.g. being on a mobile connection on the phone and then wifi gets connected.

[janlelis]

Suggestion: We implement this together with use case 1 (palavatv/palava-web#8)

[farao]

Well, it is a precondition for case 1 but still a separate feature which we could also test with case 3?

[farao]

I don't think that the code for the renegotiation will change when tackling screen sharing as a next step.

[janlelis]

Well, it is a precondition for case 1 but still a separate feature which we could also test with case 3?

True, you are right of course, case 3 only requires a mobile phone and a desktop computer to test