Keep balance between channels the same when changing volume
Previously setGenericVolume would apply the same amount of difference on all channels, making e.g. 100%/50% -> 80%/30%.
This MR changes it to keep the ratios equal instead, so that the resulting volume would be 80%/40%, keeping the balance the same.
Additionally, the VolumeSlider.qml file now deals with setting the volume by itself, so that there are no more issues with one volume slider changing the other. Not sure I'm entirely happy with the approach in that commit, so if anything could be done better, suggestions are welcome!
By the way, this is my second ever MR to a KDE project, so sorry if I did anything wrong :)