Enable wheel events on Slider {}

Summary:
So far PC3 version of the slider doesn't react to events, this change
enables it.

Test Plan: Used it with a test.

Reviewers: Plasma, broulik, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26055