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/CTextRenderBuffer.hpp

@@ -40,7 +40,7 @@ public:
         Command x4_command;
         u16 x8_xPos;
         u16 xa_zPos;
-        wchar_t xc_glyph;
+        char16_t xc_glyph;
         u8 xe_imageIndex;
     };
 #endif