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:
FIXED-IN: 5.15.1