Feature request: Increase/decrease volume message

[rugarci]

It would be nice to have 2 new control message to be able to change volume in a relative way without having to know the current state

Another option would be to accept i.e. "volume":{"muted":false,"delta": -10} in current setVolume request

[badaix]

What is the use case for this?

[rugarci]

I some environments could be difficult or tricky to make 2 calls, one for knowing the current volume level and a second one to modify it

More exactly I would use it in my HA deployment: Alexa -> NodeRed -> SnapcastServer

With Alexa, e.g is very simple to implement the command "Increase hall speakers volume" but It is not so easy to implement "Set hall speakers volume to X%"

[rugarci]

Consider also toogleMute