kcm/VolumeSlider: fix scroll
stepSize was not set to disable tickmarks. However scrolling also depends on stepSize. qqc2-desktop-stlye implements scrolling via increase()/descrease() functions, which default to steps of 0.1 when stepSize is not set.
Override them to get scrolling working without setting stepSize.
BUG: 459639 CCBUG: 409325 FIXED-IN: 6.0
Edited by Yifan Zhu