LibHTML+Browser: Show target URL of hovered links in Browser statusbar

HtmlView will now invoke the on_link_hover hook when the cursor enters
or leaves a DOM node that has an enclosing link element.

This patch also updates the meaning of Node::enclosing_link_element()
to find the nearest HTMLAnchorElementAncestor *with an href attribute*.

Libraries/LibHTML/DOM/Node.h

@@ -75,6 +75,8 @@ public:
 
     void invalidate_style();
 
+    bool is_link() const;
+
 protected:
     Node(Document&, NodeType);