Protocol: QJsonObject for tracer.

Summary:
Our former handwritter cmd->json was missing escaping strings propperly
and had ending , and further more. QJsonObject/QJsonArray are more easy
to use and have propper json output.

Test Plan: run akonadiconsole and view json.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D13104