Move values the mainmenu caches to dedicated files (#14433)

src/script/scripting_mainmenu.h

@@ -37,6 +37,9 @@ public:
 	// Global step handler to pass back async events
 	void step();
 
+	// Calls core.on_before_close()
+	void beforeClose();
+
 	// Pass async events from engine to async threads
 	u32 queueAsync(std::string &&serialized_func,
 		std::string &&serialized_param);