Allow creating Followup reminders through a DBus call to the agent

Summary:
The idea is for the client code to use this DBus method rather than writing to a shared
config file and triggering reload. This way it will be possible to get rid of the
intermediate libKF5FollowUpReminder library in kdepim-apps-libs.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29622