feat(akonadi): sort agents alphabetically

Summary:
Partially solves: https://phabricator.kde.org/T9428

This sorts alphabetically the Agents

Test Plan: Akonadi has been compiled with the patch and akonadiconsole has been launched. Everything seems to work as expected and *sorted* :-)

Reviewers: KDE PIM: Junior Jobs, dvratil, vkrause

Reviewed By: KDE PIM: Junior Jobs, dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29604