SendLaterAgent: merge libKF5SendLater library into the agent

Summary:
The library has been split into two parts: the client-side code now
lives in MessageComposer where it's used exclusively, and all the code
that is used by the Agent has been moved in here.

Instead of the client writing into a shared config file, a new DBus
method has been added to the agent that the client can call in order to
schedule a new message for sending later.

Tests for classes that now live in MessageComposer have been moved to MessageComposer.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-doc-english, kde-pim

Tags: #kde_pim, #documentation

Differential Revision: https://phabricator.kde.org/D29724