win32: convert utf-8/utf-16 when accessing win32 apis

- any functions that get strings from windows apis, are converted
  and stored as utf8 strings internally
- anytime an internal utf8 string is passed as a parameter to a
  windows api function, it is converted to a wstring
- all of these translations apply currently to file paths
- a windows client should make sure to setlocale to utf8 because
  we use some dependencies that call mbstowcs and wcstombs instead
  of WideCharToMultiByte/MultiByteToWideChar

Change-Id: Ic13f55ace491e1088c0a3d436d3a0d02df7216c3
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>