add missing interface file

include/nana/gui/detail/widget_notifier_interface.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <memory>
+#include <nana/gui/widgets/widget.hpp>
+
+namespace nana {
+namespace detail {
+
+class widget_notifier_interface {
+public:
+	virtual widget* widget_ptr() const = 0;
+	virtual void destroy() = 0;
+	virtual std::wstring caption() = 0;
+	virtual void caption(std::wstring text) = 0;
+	
+	static std::unique_ptr<widget_notifier_interface> get_notifier(widget* wdg);
+private:
+	widget::notifier* p;
+};
+
+} // namespace detail
+} // namespace nana