PIM-2288: Dispatch event after handler call

| Q                   | A
| ------------------- | ---
| Bug fix?            | N
| New feature?        | Y
| BC breaks?          | N
| Tests pass?         | Y
| Scenarios pass?     | Y
| Checkstyle issues?  | N
| Changelog updated?  | Y
| Fixed tickets       | PIM-2288
| Doc PR              | 

You have to define Mass Action Subscribers service:

```    # Mass action subscribers
    pim_datagrid.extension.mass_action.subscriber.acme:
        class: Pim\Bundle\DataGridBundle\Extension\MassAction\Subscriber\AcmeSubscriber
        tags:
            - { name: kernel.event_subscriber } ```

And define a subscriber class:

```
<?php
class AcmeSubscriber implements EventSubscriberInterface
{
    static public function getSubscribedEvents()
    {
        return array(
            MassActionEvents::MASS_ACTION_POST_HANDLER => array('onPostHandler')
        );
    }

    public function onPostHandler(MassActionEvent $event)
    {
        $datagridName = $event->getDatagrid()->getName();
        $actionName   = $event->getMassAction()->getName();

        if ($datagridName === 'product-grid' && $actionName === 'delete') {
            // TODO: Something
        }
    }
}
```