kcms/colors: Attach color scheme editor to KCM window on Wayland, too

Kai Uwe Broulikrequested to merge
work/kbroulik/colors-kcm-xdg-foreign into master

Using the new KWaylandExtras::exportWindow functionality.

Also drop the "x11" prefix from the window handle since realistically both KCM and editor will be running under the same windowing system.

@nicolasfella

Color scheme editor no longer shows up in panel and is properly parented to the KCM just like under X.

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