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