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/paint/graphics.cpp

@@ -812,7 +812,7 @@ namespace paint
 			size_.width = size_.height = 0;
 		}
 
-		void graphics::save_as_file(const char* file)
+		void graphics::save_as_file(const char* file) const
 		{
 			if(handle_)
 			{