[sendlater] Use the new agent DBus call to save SendLater information

Summary:
Rather than using a shared library from kdepim-apps-libs, we use DBus to
talk to the agent. This ultimately allows us to kill the dedicated
libKF5SendLater library.

This also moves some pieces of the libKF5SendLater library that are used
in MessageComposer public API into MessageComposer library.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29722