Fix dispatch event to work with contracts

[wbloszyk]

Subject

This PR replace event dispach parameter to work correctly with constracts.

I am targeting this branch, because this change respect BC.

Changelog

### Fixed
- Deprecations for event dispatching

[jordisala1991]

You should change: use Symfony\Component\EventDispatcher\EventDispatcherInterface; to use the contract.

[jordisala1991]

Can you require the contracts for event dispatcher on composer?

[wbloszyk]

@jordisala1991
Done. I copy it from AdminBundle. For now version ^2.0 will not be use becouse symfony/event-dispatcher require ^1.1. Anyway support for Symfony5 is close so i keep it here too.

[wbloszyk]

@core23 I update test too. Should I do something more?

[core23]

Please add a changelog entry, after that it's RTM

[wbloszyk]

@core23 Changelog can be like this?

[core23]

Thanks @wbloszyk