CCharsetConverter: add simpleWToUtf8() for safe convertion on startup/shutdown, add tests for simpleWToUtf8()

f simpleWToUtf8
CCharsetConverter: add simpleUtf8ToW() for safe convertion on startup/shutdown, add tests for simpleUtf8ToW()
[win32] Use simpleWToUtf8() and simpleUtf8ToW() in Win32Directory.cpp and Win32SMBDirectory.cpp
[win32] Use simpleUtf8ToW() and simpleWToUtf8() in win32/WIN32Util.cpp