Add walking_sound_volume_modifier for CarPrototype

[Shemp]

Currently, a "car" vehicle will always play the "walking_sound" associated with tiles when it is moving over them.

I would like a way to influence the intensity of that sound. In particular, I have mods such as Aircraft in mind whose vehicles are "floating" and shouldn't make any sounds that would imply contact with the ground.

My proposition is:
walking_sound_volume_modifier :: float
Default: 1
Must be between 0 and 1.

I don't know if having values >1 would sense on the engine side, to let vehicles intentionally have louder walking sounds than normal.

EDIT: I just noticed that walking_sound_volume_modifier exists for SpiderLegPrototype. The property on cars should behave similarly to this.

[Donion]

A Car plays the "driving_sound" of tiles as it drives over, so it would be a driving_sound_volume_modifier, but otherwise it sounds reasonable to me.

[Donion]

CarPrototype::driving_sound_volume_modifier is implemented for 2.1.

It cannot be negative, values above 1.0 are allowed.

Thanks for the suggestion.