Support simplified event dispatching

[richardhj]

See https://symfony.com/blog/new-in-symfony-4-3-simpler-event-dispatching#supporting-both-dispatchers.

Taking the PostDeleteModelEvent as an exapmle:

$event = new PostDeleteModelEvent($environment, $model);
$this->dispatcher->dispatch($event, $event::NAME);

Target: DCG 2.2 / Contao 4.9 (Symfony >=4.3)

[zonky2]

@discordier @davidmaack

is this implemented?
e.g.

dc-general/src/Contao/View/Contao2BackendView/ActionHandler/DeleteHandler.php

Lines 183 to 185 in 5c240cb

	// Trigger event after the model is deleted.
	$postDeleteEvent = new PostDeleteModelEvent($environment, $model);
	$environment->getEventDispatcher()->dispatch($postDeleteEvent, $postDeleteEvent::NAME);