LibWeb: Basic implementation of global event handlers :^)

Document and HTMLElement now inherit from HTML::GlobalEventHandlers
which allows them to support "onfoo" event handler attributes.

These are assignable both via IDL attributes and content attributes.

Event listeners constructed this way get a special "attribute" flag
on them so we know which one to replace if you reassign them.
This also allows them to coexist with EventTarget.addEventListener().

This is all a bit sloppy, but it works decently for a first cut.
The Window object should also inherit GlobalEventHandlers, but since
we don't generate it from IDL, I haven't taken that step here.

Also this would be a lot nicer if we supported IDL mixins.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -55,6 +55,7 @@ enum class QuirksMode {
 class Document
     : public ParentNode
     , public NonElementParentNode<Document>
+    , public HTML::GlobalEventHandlers
     , public Bindings::ScriptExecutionContext {
 public:
     using WrapperType = Bindings::DocumentWrapper;
@@ -233,8 +234,12 @@ public:
 private:
     explicit Document(const URL&);
 
+    // ^DOM::Node
     virtual RefPtr<Layout::Node> create_layout_node() override;
 
+    // ^HTML::GlobalEventHandlers
+    virtual EventTarget& global_event_handlers_to_event_target() final { return *this; }
+
     void tear_down_layout_tree();
 
     void increment_referencing_node_count()