Add min/max volume buffers

[SteveMicroNova]

When you change the volume of a group from 50 to 75, that is processed as adding 25 to the volume of all underlying zones. However, if you have say 1 zone at 0 and another zone at 100 (the group then being set to 50 as that's the averaged volume level), then the first zone will be set to 25 and the second will still be 100, meaning they've gotten closer due to hitting the peak volume along the way

Add buffer space to the tops and bottoms of volume values so that these delta changes maintain the same relative distance between zones

[SteveMicroNova]

I view this as a prereq to #1054 and #337