Porting the Fonts KCM to QML in 24b960f92284 set the state of the font
preview labels to disabled, resulting in a light gray text with too
little contrast to enable a usable preview of the chosen font.
In the QWidgets based KCM the following code was used:
QLabel::setFrameStyle(QFrame::StyledPanel | QFrame::Sunken)
As that property is not available from QML, we use an alternative
approach by setting the state to readOnly. To also indicate
visually that nothing can be typed into the textedit, we add
Kirigami.Theme.inherit: true which results in changing the
background from white to grey.