LibWeb: Separate "event listener" from "EventListener"

I can't imagine how this happened, but it seems we've managed to
conflate the "event listener" and "EventListener" concepts from the DOM
specification in some parts of the code.

We previously had two things:

    - DOM::EventListener
    - DOM::EventTarget::EventListenerRegistration

DOM::EventListener was roughly the "EventListener" IDL type,
and DOM::EventTarget::EventListenerRegistration was roughly the "event
listener" concept. However, they were used interchangeably (and
incorrectly!) in many places.

After this patch, we now have:

    - DOM::IDLEventListener
    - DOM::DOMEventListener

DOM::IDLEventListener is the "EventListener" IDL type,
and DOM::DOMEventListener is the "event listener" concept.

This patch also updates the addEventListener() and removeEventListener()
functions to follow the spec more closely, along with the "inner invoke"
function in our EventDispatcher.

Userland/Libraries/LibWeb/Forward.h

@@ -84,15 +84,16 @@ class Document;
 class DocumentFragment;
 class DocumentLoadEventDelayer;
 class DocumentType;
+class DOMEventListener;
 class DOMException;
 class DOMImplementation;
 class DOMTokenList;
 class Element;
 class Event;
 class EventHandler;
-class EventListener;
 class EventTarget;
 class HTMLCollection;
+class IDLEventListener;
 class LiveNodeList;
 class NamedNodeMap;
 class Node;