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

metaforce-gui/SysReqTableView.cpp

@@ -8,7 +8,7 @@
 #include <QDomDocument>
 #include <QProcess>
 #include <QStorageInfo>
-#include "FindBlender.hpp"
+#include "hecl/Blender/FindBlender.hpp"
 #include <QDebug>
 
 #if _WIN32