LibWeb: Implement getting and setting element.innerHTML

Getting the innerHTML property will recurse through the subtree inside
the element and serialize it into a string as it goes.

Setting it will parse the set value as an HTML fragment. It will then
remove all current children of the element and replace them with all
the children inside the parsed fragment.

Setting element.innerHTML will currently force a complete rebuild of
the document's layout tree.

This is pretty neat! :^)

Libraries/LibWeb/Forward.h

@@ -45,6 +45,7 @@ namespace Bindings {
 
 class CanvasRenderingContext2DWrapper;
 class DocumentWrapper;
+class ElementWrapper;
 class EventWrapper;
 class EventListenerWrapper;
 class EventTargetWrapper;