Xdg Decoration Support

Summary:
Usual massive boilerplate.

Should allow us to remove the injection in our QPT, as Qt 5.12 implements
this protocol directly. Will probably be relevant for other toolkits as
it's more standardised than our custom one.

Client is mostly 1:1 with the protocol, server side has some tweaking to
fit with kwayland turning an event based API into a state based one.

Test Plan:
Relevant unit test
Tried in modified kwin with our QPT disabled

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D17498