[EventDispatcher] Add an interface for the ContainerAwareEventDispatcher

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Currently if a class depends on the ContainerAwareEventDispatcher it's not possible to replace it by the TraceableEventDispatcher without rewriting the class.
I wanted to talk about a consistent implementation*
But yeah, I agree with you about protected vs private. Here the best visility should be package but it does not exist  PHP :) so private or protected...