Expose AbstractClient's colorScheme as a property

Summary:
This exposes colorScheme as a property which is then usable by kwin
scripts.
Wanted by T9769.

Test Plan:
Michail to test in a script.

Reviewers: #kwin, zzag, graesslin

Reviewed By: #kwin, zzag, graesslin

Subscribers: mvourlakos, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15913