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

source/gui/detail/native_window_interface.cpp

@@ -537,6 +537,30 @@ namespace nana{
 			return false;
 		}
 
+		bool native_interface::window_icon(native_window_type wd, const paint::image& big_icon, const paint::image& small_icon)
+		{
+#if defined(NANA_WINDOWS)
+			HICON h_big_icon = paint::image_accessor::icon(big_icon);
+			HICON h_small_icon = paint::image_accessor::icon(small_icon);
+			if (h_big_icon || h_small_icon)
+			{
+				nana::detail::platform_spec::instance().keep_window_icon(wd, (!big_icon.empty() ? big_icon : small_icon));
+				if (h_big_icon) {
+					::SendMessage(reinterpret_cast<HWND>(wd), WM_SETICON, ICON_BIG, reinterpret_cast<LPARAM>(h_big_icon));
+				}
+				if (h_small_icon) {
+					::SendMessage(reinterpret_cast<HWND>(wd), WM_SETICON, ICON_SMALL, reinterpret_cast<WPARAM>(h_small_icon));
+				}
+				return true;
+			}
+#elif defined(NANA_X11)
+			return window_icon(wd, big_icon);
+#endif
+			return false;
+		}
+
+
+
 		void native_interface::activate_owner(native_window_type wd)
 		{
 #if defined(NANA_WINDOWS)

source/gui/detail/window_manager.cpp

@@ -43,7 +43,8 @@ namespace detail
 				root_register	misc_register;
 				handle_manager<core_window_t*, window_manager, window_handle_deleter>	wd_register;
 				signal_manager	signal;
-				paint::image default_icon;
+				paint::image default_icon_big;
+				paint::image default_icon_small;
 			};
 		//end struct wdm_private_impl
 
@@ -199,11 +200,6 @@ namespace detail
 			}
 		}
 
-		void window_manager::default_icon(const paint::image& img)
-		{
-			impl_->default_icon = img;
-		}
-
 		bool window_manager::available(core_window_t* wd)
 		{
 			return impl_->wd_register.available(wd);
@@ -264,7 +260,7 @@ namespace detail
 					insert_frame(owner, wd);
 
 				bedrock::inc_window(wd->thread_id);
-				this->icon(wd, impl_->default_icon);
+				this->icon(wd, impl_->default_icon_big, impl_->default_icon_small);
 				return wd;
 			}
 			return nullptr;
@@ -404,6 +400,18 @@ namespace detail
 			}
 		}
 
+		void window_manager::default_icon(const paint::image& img)
+		{
+			impl_->default_icon_big = img;
+			impl_->default_icon_small = img;
+		}
+
+		void window_manager::default_icon(const nana::paint::image& big, const nana::paint::image& small)
+		{
+			impl_->default_icon_big = big;
+			impl_->default_icon_small = small;
+		}
+
 		void window_manager::icon(core_window_t* wd, const paint::image& img)
 		{
 			if(false == img.empty())
@@ -417,6 +425,19 @@ namespace detail
 			}
 		}
 
+		void window_manager::icon(core_window_t* wd, const paint::image& big_icon, const paint::image& small_icon)
+		{
+			if(!big_icon.empty() || !small_icon.empty())
+			{
+				std::lock_guard<decltype(mutex_)> lock(mutex_);
+				if (impl_->wd_register.available(wd))
+				{
+					if(wd->other.category == category::root_tag::value)
+						native_interface::window_icon(wd->root, big_icon, small_icon);
+				}
+			}
+		}
+
 		//show
 		//@brief: show or hide a window
 		bool window_manager::show(core_window_t* wd, bool visible)