Fix anchors error in AbstractKCM & SimpleKCM types

This is just the way Kirigami.Page works, and there's only so much we
can do in downstream components.

Fixes issues like the following:

  file:///usr/local/kde/share/kpackage/kcms/kcm_sddm/contents/ui/main.qml:22:13:
  QML InlineMessage: Cannot anchor to an item that isn't a parent or sibling.