[KCM] Port to QQC2

Summary:
This patch ports the KCM to QQC2, which has the side effect of working around https://bugreports.qt.io/browse/QTBUG-70481 and improving the presentation for people using fractional Qt scale factors.

BUG: 397954
FIXED-IN: 5.16.0

Test Plan: All functionality tested still worked. No visual changes at 1x scale. With a fractional scale factor, it now looks good:

Reviewers: Plasma, drosca

Reviewed By: drosca

Subscribers: apol, huftis, nicolasfella, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15304