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