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/filebox.cpp

@@ -1021,6 +1021,7 @@ namespace nana
 			
 			if (!impl_->open_or_save)
 				ofn.Flags = OFN_OVERWRITEPROMPT;	//Overwrite prompt if it is save mode
+			ofn.Flags |= OFN_NOCHANGEDIR;
 			
 			if(FALSE == (impl_->open_or_save ? ::GetOpenFileName(&ofn) : ::GetSaveFileName(&ofn)))
 				return false;