Add the config to use the system color in Korganizer (see D16377)

Summary:
see D16377 for more details

Screenshot:

Reviewers: KDE PIM, KDE PIM: Junior Jobs, mlaurent

Reviewed By: KDE PIM, mlaurent

Subscribers: lbeltrame, mlaurent, kde-pim

Tags: KDE PIM

Maniphest Tasks: T9420

Differential Revision: https://phabricator.kde.org/D16393