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/CMakeLists.txt

@@ -55,6 +55,7 @@ set(SOURCES
     HTML/AttributeNames.cpp
     HTML/CanvasRenderingContext2D.cpp
     HTML/EventNames.cpp
+    HTML/GlobalEventHandlers.cpp
     HTML/HTMLAnchorElement.cpp
     HTML/HTMLAreaElement.cpp
     HTML/HTMLAudioElement.cpp