I disagree. Here is what I have in mind:
* We add `PRE_PURCHASE` and `POST_PURCHASE` events in `SyliusCheckoutEvents` (not in sylius_payum_*)
* We pass $payment to it (which is actually already done)
* Then we keep this listener in `CoreBundle` and look at the status of the payment (`$payment = $event->getSubject()`)

So that everything is clean, and we don't mix core and payum. What do you think?