GUM Capabilities / Settings

[robin-raymond]

$related #65

More generalized reuse of the GUM style Capabilities / Settings throughout instead of re-defining specific things all over again in our own style.

Fix:

dictionary RTCRtpCapabilities {
    sequence<RTCRtpCodec> audioCodecs;
    sequence<RTCRtpCodec> videoCodecs;
    sequence<DOMString>   headerExtensions;
    Capabilities          features;
    Capabilities          rtcpFeedback;
};

dictionary RTCRtpCodec {
    DOMString     name = "";
    unsigned int? clockRate = null;
    unsigned int? numChannels = 1;
    Capabilities  formats;
};

dictionary RTCRtpCodecParameters {
    unsigned byte payloadType = null;
    RTCRtpCodec   codec;
    Settings      formats;
    Settings      rtcpFeedback;
};

Definitions will be re-used from GUM.