LibHTML: Implement naive hit testing

We don't have proper line boxes yet, so we can't easily hit test
inline text.

Libraries/LibHTML/HtmlView.h

@@ -17,6 +17,7 @@ protected:
 
     virtual void resize_event(GResizeEvent&) override;
     virtual void paint_event(GPaintEvent&) override;
+    virtual void mousemove_event(GMouseEvent&) override;
 
 private:
     void layout_and_sync_size();