Use UTF-8 exclusively internally

This removes SystemString, SystemChar, etc.
All filepaths and log strings are assumed to be UTF-8,
with conversions to UTF-16 for Windows APIs as appropriate.

Updates amuse, athena, boo, kabufua and nod

visigen/VISIRenderer.cpp

@@ -438,11 +438,7 @@ void VISIRenderer::Run(FPercent updatePercent) {
 
   ProcessType parentPid = 0;
   if (m_argc > 4)
-#ifdef _WIN32
-    parentPid = ProcessType(wcstoull(m_argv[4], nullptr, 10));
-#else
     parentPid = ProcessType(strtoull(m_argv[4], nullptr, 10));
-#endif
 
   uint32_t layer2LightCount = 0;
   {