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/MP1/CFrontEndUI.cpp

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <array>
 #include <ctime>
+#include <fmt/xchar.h>
 
 #include "NESEmulator/CNESEmulator.hpp"