Support custom callback aliases for event subscriptions

[Majkl578]

Event subscribers should be able to specify a callback alias for each registered event. Imagine this fictional scenario:

class SomeChangeListener implements Subscriber
{
    public function getSubscribedEvents()
    {
        return array(
            'User::changed',
            'Book::changed',
        );
    }
}

In this case, I end up with a conflict. It'd be essential to set aliases, like this:

        return array(
            'User::changed' => 'userChanged',
            'Book::changed' => 'bookChanged',
        );