It's kept separate in case we ever merge the server decoration manager
interface upstream, and this is too kwin specific to have in a
potentially shared spec.
Code is a copy+paste of existing boilerplate.
Replaces use of the deprecated qt extended surface.
I'd like to target 5.42 as otherwise we'll have a regression when we
release Plasma 5.12 as Qt's XDGv6 doesn't have a working qt extended