Summary:
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
surface.

Test Plan: Attached unit test

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: plasma-devel, #frameworks

Tags: #frameworks, #plasma

Differential Revision: https://phabricator.kde.org/D9599