Move ReadGamePrefsFromDisk() and SaveGamePrefsToDisk()

to StepMania.cpp.  This is to reduce the dependencies of PrefsManager:
code everywhere depends on it, but it depends on code everywhere,
resulting in lots of cross-dependencies.  (Just a small step to being able
to compile smaller bits of the engine for testing.)

stepmania/src/StepMania.h

@@ -15,6 +15,8 @@ int SMmain(int argc, char* argv[]);
 void ApplyGraphicOptions();
 void ExitGame();
 void ResetGame( bool ReturnToFirstScreen=true );
+void ReadGamePrefsFromDisk();
+void SaveGamePrefsToDisk();
 
 #if defined(WIN32)
 extern HWND g_hWndMain;