improve visualization of group caption

include/nana/gui/programming_interface.hpp

@@ -301,6 +301,8 @@ namespace API
 	void window_size(window, const size&);
 	size window_outline_size(window);
 	void window_outline_size(window, const size&);
+
+	nana::optional<rectangle> window_rectangle(window);
 	bool get_window_rectangle(window, rectangle&);
 	bool track_window_size(window, const size&, bool true_for_max);   ///< Sets the minimum or maximum tracking size of a window.
 	void window_enabled(window, bool);