Start adding implementation for Qt GUI system audio preferences:
Start adding implementation for Qt GUI system audio preferences:

* Still much more polish to do
* Volume slider appears to be logarithmic rather than linear
* Radio buttons don't persist value
* Volume slider isn't initialised to system value
* Radio buttons aren't initialised to system value
* Thanks to Zack Igielman for suggesting the specific amixer commands.
jweather/sonic-pi,jweather/sonic-pi,jweather/sonic-pi,jweather/sonic-pi,jweather/sonic-pi