[2.3][EventDipsatcher] Make events lighter

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | na
| License       | MIT
| Doc PR        | symfony/symfony-docs#2476

I've taken the previous discussions and taking into consideration @fabpot does not want to break BC. This PR now provides the `EventDispatcher` as an argument to listeners. I've made a second separate commit which also passes the event name.

In order not to break the `EventDispatcherInterface` I have not added a third argument to `EventDipsatcher::dispatch()` which would allow to solve #6686. I have a way to do that without breaking the interface but it's probably the subject of a separate PR after this would be merged or I could propose it here.