Add support for KColorScheme

Summary: This depends on D24060, so this can only go in once we depend on KF 5.63.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Maniphest Tasks: T8837

Differential Revision: https://phabricator.kde.org/D24094