onlocalcandidate issues

[aboba]

Currently the onlocalcandidate event doesn't provide the new candidates. Also, there is no indication of when no new candidates are likely (e.g. all local host, server reflexive and TURN candidates have been provided).

[aboba]

Proposed fix:

interface onlocalcandidate : Event {
readonly attribute? RTCIceCandidate candidate;
};

If candidate is null then no more local candidates are available.