Use QFormLayout for sliders

Summary:
QFormLayout is more natural for label/widget pairs and allows
to have uniform label alignment in different styles.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5886