Simplify replies connections ownership.

By using new QObject::connect signal we do not have to manually trace
QObject connections anymore.

The patch fixes random crash in EnginioModel autotests in which a not
needed anymore reply was delivered.

Change-Id: I71e21f486d8ed6b55889a1f71d6c801e20bdab09
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>