Update spacing of configuration widgets

With KF6, KPageView based widgets don't provide their own margins
anymore, so we don't need to overwrite the contentMargins of the widget
itself.

Also use flat QGroupBox when the groupbox is the only element inside the
configuration page to reduce the amount of frames.

(cherry picked from commit b22ace0ac02ccabdccf93cddf01ca0bfd14e0c6b)