[wayland] XdgDecoration Support

Summary:
Does something similar to our existing ServerDecoration, but based
around XDG Shell patterns and with a few subtle differneces.

We'll probably still need both in kwin for the forseeable future as GTK3
won't ever change from using the KDE Server Decoration.

Test Plan:
Relevant unit test. It's a bit simpler as spec states
toolkits must follow what the compositor configures if they
bind the interface.

Modified plasma-integration to remove ServerIntegration
(as Qt5.12 has native support) all my windows look and act the same.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17756