Added framework for vehicle horns

(See issue #3070)

Alright, this is a basic framework I threw together for vehicle horns. So far we have:
* Bicycle horns. They spawn on bikes, can be attached to any vehicle with just 1 skill in mechanics, and make a sound about half as loud as the noise emitter. If you remove it from a vehicle you can 'a'ctivate it to honk it, otherwise it's in the ^ menu.
* Car horns. They require 3 skill in mechanics to wire up to a vehicle, cannot be 'a'ctivated manually, and make as much noise as a noise emitter.
* Truck horns. See below.

TODO:
* ~~Right now all cars use the same horn. Semis should probably use a louder one.~~ Done!
* Car horns should probably require a power source in the car. Right now they can be installed in bikes.
* Use car horns in crafting recipe for noise emitters?

Any other ideas/suggestions?