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

Runtime/GuiSys/CErrorOutputWindow.hpp

@@ -16,7 +16,7 @@ private:
   bool x18_26_;
   bool x18_27_;
   bool x18_28_;
-  const wchar_t* x1c_msg;
+  const char16_t* x1c_msg;
 
 public:
   explicit CErrorOutputWindow(bool);