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