LibHTML: Implement basic HTMLElement.title support

We now show a tooltip for the hovered node's enclosing HTML element's
title attribute, if one is present.

This patch also adds HTMLHeadingElement. The tags h1-h6 will now create
the right kind of objects.

Libraries/LibHTML/DOM/HTMLElement.h

@@ -6,4 +6,9 @@ class HTMLElement : public Element {
 public:
     HTMLElement(Document&, const String& tag_name);
     virtual ~HTMLElement() override;
+
+    String title() const { return attribute("title"); }
+
+private:
+    virtual bool is_html_element() const final { return true; }
 };