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