kcm/ui: add required id and parentLayout property

For correct alignment with wallpaper config, an ancestor node must have
id "appearanceRoot" and property "parentLayout". See the files
plasma-workspace/wallpapers/image/
- imagepackage/contents/ui/config.qml
- slideshowpackage/contents/ui/SlideshowComponent.qml

BUG: 478599
FIXED-IN: 6.0