[transfer-engine] Ensure group notification is published with the correct preview body.
MNotificationGroup::publish() sets its x-nemo-preview-body field using the preview body of the last added notification group. This means if the group is published before the notification, the preview body used will be that of the last published notification group instead of the current one. If the group is published after the notification instead, the notification will ensure the group properties are set correctly before the group is published.
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