Add ability to change volume

[gauravahuja]

Some monitors have built in speakers and it would be good to control volume also.

[oscfdezdz]

ddcutil vcpinfo output:

...

VCP code 62: Audio speaker volume
   Adjusts speaker volume
   MCCS versions: 2.0, 2.1, 3.0, 2.2
   ddcutil feature subsets: AUDIO
   Attributes (v2.0): Read Write, Continuous (normal)
   Attributes (v2.1): Read Write, Continuous (normal)
   Attributes (v3.0): Read Write, Non-Continuous with continuous subrange
   Attributes (v2.2): Read Write, Non-Continuous with continuous subrange

...

The implementation could be very similar to the one that already exists for modifying brightness.

[daitj]

@oscfdezdz Yes functionality wise you basically need to change the icons and setvcp code 62 instead of 10, but adding this to settings and creating proper UI so they look nice is where most of the time will go.