Slider: Fix slider scrolling direction

We always want the slider to increase its value when scrolling up or right. This corresponds to positive values on the Y axis and negative values on the X axis.

This is independent of the inverted scrolling setting.

Note that the Qt documentation on WheelEvent is dubious: On https://doc.qt.io/qt-5/qml-qtquick-wheelevent.html, the angleDelta section suggest that positive is up/right, but the inverted paragraph mentions up/left.

This works consistently for me on every QML slider using the style (but plasma of course) and can be easily tested on the Mouse KCM. This one has the advantage of allowing to select the "invert scrolling" setting to test all of the options.