applet: fix unable to finetune slider

The internal value of a slider always changes by multiples of stepSize,
regardless of the value of snapMode, so setting snapMode does not work.
Instead, implement custom wheel event handling.

Also fixes horizontal scrolling behavior when invert scroll direction is
disabled.

This commit borrows from Slider.qml and
5d6d74ef68889da7400cb2a16a73527f1927424d in qqc2-desktop-style.

BUG: 479028


(cherry picked from commit 50fe4f1f)