[2.3][EventDispatcher] Make events lighter
merged branch drak/edaware2 (PR #7852)

This PR was squashed before being merged into the master branch (closes #7852).

Discussion
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[2.3][EventDispatcher] Make events lighter

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| 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.

This PR is alternative to #7582

Commits
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e2bff32 [2.3][EventDispatcher] Make events lighter
[EventDispatcher] fixed some phpdoc