Made open method work for UTF8 paths on windows

Fixed: The Open method in DefaultIOSystem did not work for UTF8-Paths in windows

Problem: 
On windows platform, the Open method did only work for ANSI-compatible file paths, e.g. failed for pathnames with german umlauts, chinese characeters etc.

Solution:
srcFile string is now decoded from UTF8 to wstring. For paths that worked before, it makes no difference. But UTF8-paths that failed before will now work as well, because they are called as wstring to _wfopen, so that windows understands them.