New: Add coupler stretch / compress sounds

[leezer3]

This PR adds sounds played when the couplers stretch and compress. These are ignored when the coupler distance is 0.

Sound.cfg:

[Coupler]
Stretch = file.wav
Compress = file.wav

Sound.xml

<Coupler>
    <Stretch>
        <FileName>file.wav</FileName>
        <Position>0,0,5</Position>
        <Radius>2.0</Radius>
    </Stretch>

    <Compress>
        <FileName>file.wav</FileName>
        <Position>0,0,5</Position>
        <Radius>2.0</Radius>
        </Compress>
</Coupler>

Flaws:

• The default coupler stretch / compress distance is 5cm- In a typical train, the slack is taken up almost instantly, and it doesn't sound perfect.
• We don't currently simulate unbraked cars, which means that the compress sound is only really heard as the train comes to a stand.
• Using the default minimum radius, we can still hear more coupler sounds than I'd like. Perhaps drop the radius to 0.5?

[OpenBveTrainSim]

@leezer3 Coupler sounds during acceleration\deceleration now seem a bit exagerated\amplified\unrealistic on some trains. Thanks OBTS