add methods to get/set sortability of listbox

include/nana/gui/widgets/listbox.hpp

@@ -552,7 +552,10 @@ By \a clicking on a header the list get \a reordered, first up, and then down al
 			_m_ease_key(&key);
 		}
 
-		            ///Sets a strict weak ordering comparer for a column
+		bool sortable() const;
+		void sortable(bool enable);
+		
+		///Sets a strict weak ordering comparer for a column
 		void set_sort_compare(size_type col, std::function<bool(const nana::string&, nana::any*,
 				                                        const nana::string&, nana::any*, bool reverse)> strick_ordering);