port the look and feel kcm to new design
Summary:
use the new design for the look and feel kcm, together with
a massive code simplification
Test Plan: pply does work, as preview of themes
Reviewers: Plasma, VDG, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D12090