add an overloading function of listbox::scroll

clicking horizontal barre moves "one letter"

include/nana/gui/widgets/listbox.hpp

@@ -558,6 +558,7 @@ By \a clicking on one header the list get \a reordered, first up, and then down
 
 		/// Scrolls the view to the first or last item of a specified category
 		void scroll(bool to_bottom, size_type cat_pos = ::nana::npos);
+		void scroll(bool to_bottom, const index_pair& pos);
 
         /// Appends a new column with a header text and the specified width at the end, and return it position
         size_type append_header(nana::string header_text, unsigned width = 120);