add OFN_NOCHANGEDIR flag to OPENFILENAME::Flags in nana::filebox::show

http://blogs.msdn.com/b/oldnewthing/archive/2010/11/12/10089878.aspx
	setting OFN_NOCHANGEDIR flag to OPENFILENAME::Flags is effective for GetOpenFileName API on Windows7.
add const qualifier to nana::paint::graphics::save_as_file
add support for small icon on Windows.
	The system displays the large icon in the ALT+TAB dialog box, and the small icon in the window caption.
	https://msdn.microsoft.com/en-us/library/windows/desktop/ms632643%28v=vs.85%29.aspx

include/nana/gui/detail/native_window_interface.hpp

@@ -45,6 +45,8 @@ namespace detail
 		static void enable_dropfiles(native_window_type, bool);
 		static void enable_window(native_window_type, bool);
 		static bool window_icon(native_window_type, const paint::image&);
+		// (On Windows) The system displays the large icon in the ALT+TAB dialog box, and the small icon in the window caption.
+		static bool window_icon(native_window_type, const paint::image& big_icon, const paint::image& small_icon);
 		static void activate_owner(native_window_type);
 		static void activate_window(native_window_type);
 		static void close_window(native_window_type);

include/nana/gui/detail/window_manager.hpp

@@ -112,7 +112,6 @@ namespace detail
 		void signal_fire_caption(core_window_t*, const nana::char_t*);
 		nana::string signal_fire_caption(core_window_t*);
 		void event_filter(core_window_t*, bool is_make, event_code);
-		void default_icon(const nana::paint::image&);
 
 		bool available(core_window_t*);
 		bool available(core_window_t *, core_window_t*);
@@ -133,7 +132,10 @@ namespace detail
 		//@brief:	Delete window handle, the handle type must be a root and a frame.
 		void destroy_handle(core_window_t*);
 
+		void default_icon(const paint::image&);
+		void default_icon(const paint::image& big_icon, const paint::image& small_icon);
 		void icon(core_window_t*, const paint::image&);
+		void icon(core_window_t*, const paint::image& big_icon, const paint::image& small_icon);
 
 		//show
 		//@brief: show or hide a window

include/nana/gui/programming_interface.hpp

@@ -120,7 +120,10 @@ namespace API
 	}
 
 	void window_icon_default(const paint::image&);
+	void window_icon_default(const paint::image& big_icon, const paint::image& small_icon);
 	void window_icon(window, const paint::image&);
+	void window_icon(window, const paint::image& big_icon, const paint::image& small_icon);
+	
 	bool empty_window(window);		///< Determines whether a window is existing.
 	bool is_window(window);			///< Determines whether a window is existing, equal to !empty_window.
 	void enable_dropfiles(window, bool);

include/nana/paint/detail/image_bmp.hpp

@@ -75,6 +75,12 @@ namespace nana{	namespace paint
 				this->close();
 			}
 
+			bool open(void* buff, size_t sz)
+			{
+				// TODO: read a BMP file from memory
+				return false;
+			}
+
 			bool open(const nana::char_t* filename)
 			{
 				if(nullptr == filename) return false;

include/nana/paint/detail/image_ico.hpp

@@ -23,6 +23,7 @@ namespace nana{	namespace paint
 			image_ico(bool is_ico);
 
 			bool open(const nana::char_t* filename);
+			bool open(void* buff, size_t sz);
 			bool alpha_channel() const;
 			bool empty() const;
 			void close();

include/nana/paint/detail/image_impl_interface.hpp

@@ -16,6 +16,7 @@ namespace nana{	namespace paint{
 		typedef nana::paint::graphics& graph_reference;
 		virtual ~image_impl_interface() = 0;	//The destructor is defined in ../image.cpp
 		virtual bool open(const nana::char_t* filename) = 0;
+		virtual bool open(void* buff, size_t sz) = 0; // reads image from memory
 		virtual bool alpha_channel() const = 0;
 		virtual bool empty() const = 0;
 		virtual void close() = 0;

include/nana/paint/graphics.hpp

@@ -130,7 +130,7 @@ namespace nana
 			void setsta();      ///<  	Clears the status if the graphics object had been changed
 			void set_changed();
 			void release();
-			void save_as_file(const char*);
+			void save_as_file(const char*) const;	// saves image as a bitmap file
 
 			void set_color(const ::nana::color&);
 			void set_text_color(const ::nana::color&);

include/nana/paint/image.hpp

@@ -37,6 +37,7 @@ namespace paint
 		image& operator=(const image& rhs);
 		image& operator=(image&&);
 		bool open(const nana::string& filename);
+		bool open_icon(void* buff, size_t sz);	// opens a icon from memory
 		bool empty() const;
 		operator unspecified_bool_t() const;
 		void close();