Improve Desktop/Plasma Theme KCM

Summary:
Similar to D18206.
The Desktop/Plasma Theme KCM has some inconsistencies and doesn't always apply the KDE HIG, so this fixes it, makes some other improvements and renames the KCM to "Plasma Theme".

Test Plan: Open the Desktop/Plasma Theme KCM

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, davidedmundson, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18247