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~~
Probably redefine this one in terms of calling the is_part_on() method too, nice to have it only check things in one place.