Added framework for vehicle horns

[Gatleos]

(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?

[atomicdryad]

Please put new enum entries at the end; If they're put in the middle it breaks save compatibility. This won't be an issue sooon

[illi-kun]

Wow, you're so fast! Good job!

[CinghialeCdda]

Semis use big airhorns, painfully loud up close. They should almost be as loud as pistol shot.

[Gatleos]

Alright, truck horns (which are only used by cargo trucks and semis) are louder than a sound described as a "disturbing scream". So that should do it. :p

The vehicle horns were moved to the end of the parts enum, and I cleaned up the code a bit. Horn honking is now in its own function, so horns can potentially be honked by something other than the player (zombies crawling through vehicle interiors?)

I should probably add in support for multiple horns, maybe making their sounds overlap to produce a louder one. I'll need to think about that. But this is in a merge-able state right now.

[atomicdryad]

This may have been put into conflict with a pr I made earlier, do you mind if I correct while merging?