LibHTML: Detect hovering over links

HtmlView now calls Node::enclosing_link_element() to find the nearest
ancestor <a> element.

This patch also adds HTMLElement and HTMLAnchorElement.

Libraries/LibHTML/DOM/HTMLAnchorElement.cpp

@@ -0,0 +1,10 @@
+#include <LibHTML/DOM/HTMLAnchorElement.h>
+
+HTMLAnchorElement::HTMLAnchorElement(Document& document, const String& tag_name)
+    : HTMLElement(document, tag_name)
+{
+}
+
+HTMLAnchorElement::~HTMLAnchorElement()
+{
+}