Fix wrong connect

I guess at some point this was wrongly ported, the slot object has to be parent(), that is a RecipientsEditor, not parent, which is the widget the new line is immediately created on.

QObject::connect: Cannot connect MessageComposer::RecipientLineNG::addRecipient(RecipientLineNG\*,QString) to (null)::addRecipient(RecipientLineNG\*,QString) is gone when starting to write a new email

REVIEW: 125477