Toggle engines

Adds ability to toggle individual engines in vehicle control menu, when more than one engine present. Choice brings up submenu listing all engines. Velocities, noise and fuel usage update accordingly.
If velocity after toggling is higher than the new safe velocity, the cruise control speed is lowered.

~~TODO: make radiators only work if the engine they are attached to is enabled~~
Especially if you do the above, you probably want one method that expects a part index that does the real work, and another method that expects the engine index and just passes the part index to the other one.