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/Input/CKeyboardMouseController.hpp

@@ -7,7 +7,7 @@ namespace metaforce {
 
 struct CKeyboardMouseControllerData {
   std::array<bool, 256> m_charKeys{};
-  std::array<bool, 26> m_specialKeys{};
+  std::array<bool, static_cast<size_t>(boo::ESpecialKey::MAX)> m_specialKeys{};
   std::array<bool, 6> m_mouseButtons{};
   boo::EModifierKey m_modMask = boo::EModifierKey::None;
   boo::SWindowCoord m_mouseCoord;