Port Archive mail agent to new config API

Summary:
The new API allows us to open agent configuration dialogs inside of the
application process that wants to show the dialog instead of having the
configuration dialogs running inside of the Akonadi agent process.

This is necessary for single-process Akonadi (running the Server and
Agent within a single process in multiple threads) and for Wayland as
well as we can no longer control placment of the configuration window
on top of other process' window.

Reviewers: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14902