Alexander Lohnau authored Dec 29, 2020 and Nate Graham committed Jan 15, 2021

The QWidgets components are about to be deprecated
and the QtQuick dialog has a bunch of visual improvements.

Considering that most KCMs already use the QtQuick components
this MR improves the consistency.

See frameworks/knewstuff!80
for the MR which introduced the non-deprecated dialog class.