Improve the Look and Feel KCM

Summary:
The LnF KCM has some inconsistencies and doesn't always apply the KDE HIG, so this fixes it and makes some other improvements.

Test Plan: Open the LnF KCM

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, rooty, rizzitello, abetts, filipf, VDG, plasma-devel, Plasma

Tags: Plasma

Maniphest Tasks: T10273

Differential Revision: https://phabricator.kde.org/D18206