[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