kcm: Use more conventional parent/background for OverlaySheet

Parenting to the KCM itself rather than its parent prevents the view
from disappearing while the OverlaySheet is open, which looks a but odd
and generated a bug report. Let's set the parent to the KCM so that the
rest of the UI beneath the dialog remains visible, as we see in other
software that uses the Kirigami.OverlaySheet component.

BUG: 442563
FIXED-IN: 5.23