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
Now you see the KCM's controls while the sheet is open: