Save calendar colors immediately rather than on exit.

Summary: They were lost if the app would crash on exit...

Test Plan: changing a calendar color and watching ~/.config/eventviewsrc be updated immediately

Reviewers: smartins, mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, dkurz, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7387