Merge contents of libKF5FollowupReminder into the Agent

Summary:
The library is no longer used anywhere and the agent now has DBus API
that clients can use to interact with it, so we no longer need a
dedicated library. This changes moves the code from the library into the
Agent (where it is still used). Some cleanup changes will follow
soon after.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29699