Slider: Fix slider scrolling direction

We always want the slider to increase its value when scrolling
up or right, no matter the inverted scrolling setting.

For "regular" scrolling, this corresponds to positive values on
the Y axis and negative values on the X axis. For "natural"
scrolling (indicated by the `inverted` property) is the opposite
so we need to reverse the sign.

Note that the Qt documentation on WheelEvent is dubious:
On https://doc.qt.io/qt-5/qml-qtquick-wheelevent.html, the
`angleDelta` section suggests that positive should be up/right,
but the `inverted` section indicates the above mentioned behavior.

BUG: 459624
FIXED-IN: 5.91.0