Eventviews: simplify/modernize resource color code

Summary:
This commit does not change behaviour in any way (except the behaviour
of the reader, who can now understand what he/she is reading)

Test Plan: Restarting korganizer, colors unchanged.

Reviewers: dvratil, ochurlaud

Reviewed By: ochurlaud

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D16507