[WIP][payment] state changed event.

This PR adds a new event `SyliusPaymentsEvent::POST_STATE_CHANGED`. I am gonna reuse this event later while working on https://github.com/Sylius/Sylius/issues/458, https://github.com/Sylius/Sylius/issues/457.

Open questions:

* Should we pass previous state as well? I think yes but in this case we have create custom event object.
* Event name can be adjust. I follow the rule `PRE_XXX`, `POST_XXX` I saw in `SyliusOrderEvents` class. But here I dont think this prefix is needed so simple `STATE_CHANGED` looks ok to me.