add missing header file

include/nana/gui/detail/widget_notifier_interface.hpp

@@ -0,0 +1,39 @@
+/*
+ *	Widget Notifier Interface
+ *	Nana C++ Library(http://www.nanapro.org)
+ *	Copyright(C) 2003-2015 Jinhao(cnjinhao@hotmail.com)
+ *
+ *	Distributed under the Boost Software License, Version 1.0.
+ *	(See accompanying file LICENSE_1_0.txt or copy at
+ *	http://www.boost.org/LICENSE_1_0.txt)
+ *
+ *	@file: nana/gui/detail/widget_notifier_interface.hpp
+ */
+
+#ifndef NANA_GUI_DETAIL_WIDGET_NOTIFIER_INTERFACE_HEADER
+#define NANA_GUI_DETAIL_WIDGET_NOTIFIER_INTERFACE_HEADER
+#include <memory>
+#include <string>
+
+namespace nana
+{
+	class widget;	//forward declaration
+
+	namespace detail
+	{
+		class widget_notifier_interface
+		{
+		public:
+			virtual ~widget_notifier_interface() = default;
+
+			static std::unique_ptr<widget_notifier_interface> get_notifier(widget*);	//defined in nana/gui/widgets/widget.cpp
+
+			virtual widget* widget_ptr() const = 0;
+			virtual void destroy() = 0;
+			virtual std::wstring caption() = 0;
+			virtual void caption(std::wstring) = 0;
+		};
+	}
+}
+
+#endif