Simplify text regarding event handlers

[ibc]

This event handler, of event handler type xxxxxx, uses the RTCXxxXxxxXxxxxEvent interface. It must be supported by all objects implementing the RTCXxxXxxxx interface.

Do we really need such an exact text in the definition of all the event handlers within the spec? Yes, WebRTC 1.0 does it, but it becomes really annoying/boring for the reader (IMHO).

[aboba]

WebRTC 1.0 has changed much of the text relating to event handlers, so that a lot of the above text is now gone. The ORTC specification should do the same.

[aboba]

Went through all the event handler text. With the adoption of the WebRTC 1.0 "statechange" event, quite a bit of the event handler text could be deleted. Also, I do not think it makes sense for an EventHandler to be nullable. The event table also needed to be updated to reflect the "statechange" event.