Use UTF16 versions of Windows API functions

[peci1]

Most of ign-common is using the ASCII versions of Windows API functions. These should be converted to the ones using wchar_t (UTF16) strings. The ASCII API is prone to text encoding problems, whereas the UTF16 API doesn't have an issue.

This would probably also deserve a helper function to convert std::string<char> to std::wstring (std::string<wchar_t>). https://stackoverflow.com/questions/4804298/how-to-convert-wstring-into-string might be a good start for this function, as it really isn't as trivial as one would think.

[peci1]

A part of the usages can be completely eliminated by switching to C++17 filesystem library. The main blocker would probably be XCode 10, which does not support filesystem (although it supports a big part of C++17, filesystem is not there - https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes#New-Features ).