UI: Add audio meter decay rate option

The decay rate of the audio meters can now be selected in the audio
settings.  The values are:

- "Fast"   (OBS default, 40 dB / 1.7s)
- "Medium" (Type I PPM,  20 dB / 1.7s)
- "Slow"   (Type II PPM, 24 dB / 2.8s)

Closes jp9000/obs-studio#1143

UI/volume-control.hpp

@@ -253,4 +253,6 @@ public:
 
 	QString GetName() const;
 	void SetName(const QString &newName);
+
+	void SetMeterDecayRate(qreal q);
 };