The ContainerAwareTraceableEventDispatcher class was tied to both the
Symfony container and the HttpKernel profiler. It made it non reusable
in another context.
The new TraceableEventDispatcher only keeps the HttpKernel profiler
integration and is able to wrap any other event dispatcher. It makes it
reusable in frameworks using the Symfony HttpKernel component like
Silex.
The only drawback is that we don't have access to the listener
priorities in the collected data anymore (but the listeners are still
ordered correctly). The change is still worth it I think.
