Quick Controls ScrollBar should have support for arrow buttons

In the windows native style, a scrollbar is expected to have arrow buttons on each side of the scrollbar groove. QQuickScrollBar doesn't have the concept of these arrow buttons (since its primary objective was touch), but in order for windows native style to work, an API should be added for those (probably a bit similar to the "up/down indicator" properties on QQuickSpinBox).

The status today is that the scrollbar looks fine - arrow buttons are shown, but you don't really intereact with them. However, they seem to almost "work", but that's just a coincidence: If I press the arrow button area now, it assumes I'm pressing the groove, so the slider handle just moves to one of the edges close to where you clicked.