kcm: Fix the load of wallpaper kcms
Wallpaper kcms use sometimes the Plasmoid object (conditionally) to check some parameters.
Recently, we ported from the old "plasmoid" context to the Plasmoid attached property coming from org.kde.plasma.plasmoid. This needs to have its import available otherwise QML rightly refuses to load anything.
This includes the import by only registering a dummy empty object so it doesn't complain anymore.