Put volume change logic centrally in VolumeSlider

This commit puts the volume change logic in one place, to prevent
issues related to changing the volume twice.

Without this commit there are issues such as dragging from 100%/100% to
100%/50% and then the mixer changing the volume to 50%/25%.

I'm not sure I'm entirely happy with this, but it seems to work.


(cherry picked from commit fec28d12)