Implement scroll and drag adjustment of values for SpinBox control

Summary:
Currently adjusting the value by scroll and click/touch+drag are not implemented.
This patch implements them.

Test Plan:
As far as I can tell, the PlasmaComponents3.Spinbox is not actually used anywhere in Plasma, so test with D29535:

Reviewers: VDG, Plasma, ahiemstra

Reviewed By: ahiemstra

Subscribers: ahiemstra, abetts, ndavis, cblack, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T9460

Differential Revision: https://phabricator.kde.org/D29534