Use std::filesysytem to remove deprecated std::codecvt_utf8_utf16

[yusuke-tokuyoshi]

std::codecvt_utf8_utf16, that was deprecated due to security reasons, is used in utf8_to_wstring.

Orochi/Orochi/OrochiUtils.cpp

Line 39 in e8da229

inline std::wstring utf8_to_wstring( const std::string& str )

If utf8_to_wstring is used only for paths, we can replace these path types with std::filesystem that handles character encoding.