Add explicit protocol for colour scheme management
Summary:
To replace the deprecated/broken qt extended surface
BUG: 385214
Test Plan: Ran test in plasma-integration
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: plasma-devel, kwin, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D9660