Support volume attenuation

[Kernald]

On some models at least (I have an rx-v579), mute has three states: On, Off and Att -20 dB. I don't know exactly how it could be handled, but in home-automation context, it could be really useful (attenuate sounds when a phone call/notification is incoming…).

[wuub]

I'll take a look :)

[sdague]

My guess is that it's the Volume,Scale method (based on what's in the v579 desc.xml that's not in my v675 desc.xml) https://github.com/sdague/rxv/blob/aaa1905a677afbc3660b1b05a520606b0de8a695/tests/samples/rx-v579-desc.xml#L519

Over the API it looks like it does this with a variable for the scaling - https://github.com/sdague/rxv/blob/aaa1905a677afbc3660b1b05a520606b0de8a695/tests/samples/rx-v579-desc.xml#L181-L191