Accept partial line scrolls

QAbstractSlider might register and use small scroll events that would
scroll less than a single line. Since we consume the scroll-event we
should accept it, so it doesn't scroll other widgets too.

Task-number: QTBUG-49549
Change-Id: I7c64c5f6cae46f02ba21058abbecb791fc3c88eb
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>