add a native wstring version textbox::append

added a textbox::append which accepts texts in std::wstring

include/nana/gui/widgets/textbox.hpp

@@ -185,7 +185,7 @@ namespace nana
 
         /// Appends an string. If `at_caret` is `true`, the string is inserted at the position of caret, otherwise, it is appended at end of the textbox.
 		textbox& append(const std::string& text, bool at_caret);
-
+        textbox& append(const std::wstring& text, bool at_caret);
 		/// Determines whether the text is line wrapped.
 		bool line_wrapped() const;
 		textbox& line_wrapped(bool);