Implement triple control using Emesary + PAX view.

[Megaf]

FlightGear now has this fantastic protocol called Emesary, so let's use it.
Triple control would be the Pilot In Command + First Office + Engineer.

Both Pilot In Command and First Officer will have cockpit view and will be able to control throttle and flight controls.
Engineer will not be able to control (send) flight controls input.

We can keep or rewrite the current PAX view.