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
