[WebRTC] Expose more of the WebRTC API.

Add get_gathering_state() returning the iceGatheringState of the
connection.

Add get_signaling_state() returning the signalingState of the
connection.

Improve JS library.

modules/webrtc/webrtc_peer_connection_extension.h

@@ -53,6 +53,8 @@ public:
 
 	/** GDExtension **/
 	EXBIND0RC(ConnectionState, get_connection_state);
+	EXBIND0RC(GatheringState, get_gathering_state);
+	EXBIND0RC(SignalingState, get_signaling_state);
 	EXBIND1R(Error, initialize, Dictionary);
 	EXBIND0R(Error, create_offer);
 	EXBIND2R(Error, set_remote_description, String, String);