[PotD] Port mostly to QQC2

Summary:
Port the Config UI mostly to QQC2, with the exception of the ComboBox,
which still suffers from a visual bug (see the inline comment in the code)

This also 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: 404248
FIXED-IN: 5.15.2

Test Plan: No change at 1x scale. Looks better at 1.2x scale:

Reviewers: Plasma, davidedmundson, kossebau, filipf

Reviewed By: filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18975