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/HTML/HTMLBodyElement.cpp

@@ -27,6 +27,7 @@
 #include <LibWeb/CSS/StyleProperties.h>
 #include <LibWeb/CSS/StyleValue.h>
 #include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/Window.h>
 #include <LibWeb/HTML/HTMLBodyElement.h>
 
 namespace Web::HTML {
@@ -78,4 +79,10 @@ void HTMLBodyElement::parse_attribute(const FlyString& name, const String& value
     }
 }
 
+DOM::EventTarget& HTMLBodyElement::global_event_handlers_to_event_target()
+{
+    // NOTE: This is a little weird, but IIUC document.body.onload actually refers to window.onload
+    return document().window();
+}
+
 }