Dialog: Always use an overlay as visual parent
If applicationWindow() is not available, fall back to the attached Overlay.overlay item as default. This might stop existing on teardown, so add a few checks to avoid warnings when the overlay becomes null.
This makes dialogs show up within System Settings KCMs even when the original parent (e.g. a Loader) has zero height or width.
Second attempt at fixing plasma/kscreenlocker!215 (comment 923985).