[followupreminder] Use the new agent DBus call instead of libKF5FollowUpReminder

Summary:
Use the new Follow-up Reminder agent DBus function to add a new reminder,
rather than using the intermediate library to write into the shared config.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29629