Factor out coloring from AgendaItem::paintEvent.

Summary:
The paintEvent method is already quite long, and calculating colors
inline prohibits a lot of const uses. This commit factors it out into
easily comprehensible small new methods, also making it more obvious
where to look for agenda item coloring for future changes.

Test Plan:
I tried all color settings for Agenda View and could not
find different behavior with or without patch -- just as expected, as
is a refactoring patch

Reviewers: KDE PIM, dvratil

Reviewed By: KDE PIM, dvratil

Subscribers: dvratil, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D16089