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/MainWindow.cpp

@@ -65,7 +65,7 @@ const QStringList MainWindow::skUpdateTracks = {QStringLiteral("stable"), QStrin
 MainWindow::MainWindow(QWidget* parent)
 : QMainWindow(parent)
 , m_ui(std::make_unique<Ui::MainWindow>())
-, m_fileMgr(_SYS_STR("metaforce"))
+, m_fileMgr("metaforce")
 , m_cvarManager(m_fileMgr)
 , m_cvarCommons(m_cvarManager)
 , m_heclProc(this)