This also separates out concerns better - UI elements just need to poke
getCallForRoom rather than care if the thing they are displaying is a
true 1:1 for this room ID or actually a conf room.
CallView is the container for either VideoViews or WaveformViews. All UI
elements listen for 'call_state' payloads and then call
CallHandler.getCall(roomId) to extract the current MatrixCall for that room.
We can't do this via stateful dispatches because dispatching does not preserve
ordering empirically (probably due to setTimeout).
