PIM-2871 : dispatch event when create/edit a category

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | Y (partially)
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?   | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-2871 (partially)
| Doc PR               |
I'd provide such shortcut in the controller that receives the dispatcher in constructor:

``` php
public function dispatch($eventName, Event $event)
{
    return $this->eventDispatcher->dispatch($eventName, $name);
}