webrtcbin do not signal when ICE candidate gathering is done

Submitted by Daniel F

Link to original bug (#794798)

Description

New webrtcbin component has signal add-ice-candidate which is sent for every discovered ICE candidate. However now there is no way to tell when all of them are gathered. I need this to send all candidates appended to end of SDP (in my case they are for local IP address only).

Please either send signal on-ice-candidate one more time with NULL argument (like https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/onicecandidate says), or add new signal - e.g. candidate-gathering-done, as in NiceAgent https://nice.freedesktop.org/libnice/NiceAgent.html

It will be beneficial to fix add-ice-candidate signal too, so GStreamer could fail faster if none of provided ICE candidates can be used. Without this it can only use timer to decide when to fail.

Version: 1.14.0