Add an "Authoritative" vs "Relative" volume change setting to groups #1079
Description
Currently, groups commit authoritative volume changes, that is they forcibly change all underlying zones to be the same volume value. This is good for when a group is a series of zones that are actually one room as this makes rooms with one zone and rooms with multiple zones have the same overall control experience, but it can make less sense when you have an "Everywhere" style group
Add a toggle to set individual groups to have relativistic volume controls in the group settings, that way you can match how a source/stream handles volume if you so choose.
This could potentially remove the need of solving #1076