New global volume management

[albaintor]

New global volume management
Currently when you enable additional output devices to stream audio to, the volume cannot be set on these devices.
Besides the main volume may not be modified at all if the AppleTV is connected to a HT receiver and will remain to 0.

With this modification, we can now set volume by level and also apply this volume level to all connected devices.
Also the reported volume reflects the average sum of all volume levels like in the notification center of the iOS device (see capture below).

Modifications applied :

1. Added support to set volume with a given level (features updated)
2. The volume is applied to all connected devices unless the user uncheck it in the setup flow/reconfigure flow. By default this new setting is enabled and can be disabled by user
3. The reported volume level is now updated with an average value of all volume levels (if option enabled), otherwise main volume only like today

I was able to test it successfully on my setup:

1. Select at least one additional device to stream audio to : AppleTV + homepod for ex
2. Set the volume level to any value (eg 50)
3. The volume is updated to all connected devices including the homepod

See the following capture :

This PR will have to be updated later once the other PR on pyatv will be merged : postlund/pyatv#2673

[zehnm]

This PR will have to be updated later once the other PR on pyatv will be merged : postlund/pyatv#2673

Shall we wait or would you rather have this PR merged already?

[albaintor]

I was hoping that @postlund would merge it quickly but I have no news since.
I think we can merge it and I will update it later once merged on pyatv side

[zehnm]

Ok, I'll test and merge it.

[albaintor]

Thanks !

[zehnm]

Looks good, thanks!

Only tested with an external HDMI output and still works the same.

[albaintor]

Great you will have to trust me on this one 😀