Use a settings object for the main settings

This unifies the settings APIs.

This also unifies the sync and async registration APIs, since the async
registration API did not support adding non-functions to the API table.

src/script/scripting_mainmenu.h

@@ -44,6 +44,7 @@ public:
 			const std::string &serialized_params);
 private:
 	void initializeModApi(lua_State *L, int top);
+	static void registerLuaClasses(lua_State *L, int top);
 
 	AsyncEngine asyncEngine;
 	DISABLE_CLASS_COPY(MainMenuScripting);