Add functions to convert to and from Windows style paths to QDir, even on Unix

Sometimes it is necessary to handle Windows style paths on UNIX like systems. QDir has toNativeSeparators(), fromNativeSeparators() and cleanPath() functions that do this kind of conversions compatible with the current platform. However, sometimes these same conversion could be needed to convert Windows style paths elsewhere, so there could be new functions to convert Windows style paths to native paths (fromWindowsToNativeSeparators()/fromNativeToWindowsSeparators() for example).