add a "level" api that has the same meaning as in the Kirigami Heading component,
This makes easier for QWidget apps to follow the typographi HIG at
https://www.my-scratch.de/HIG/style/typography.html
QWidget and QML apps will look more similar and config dialogs
the same.
together other diffs introduces more consistency within SystemSettings
between QWidget and QML based kcms