JANITORIAL: Rename EVENT_RTL

Now that ScummVM is adding RTL support, the EVENT_RTL should be disambigious that it is for returning to launcher

gui/about.cpp

@@ -473,7 +473,7 @@ void EE::run() {
 		while (g_system->getEventManager()->pollEvent(event)) {
 			switch (event.type) {
 			case Common::EVENT_QUIT:
-			case Common::EVENT_RTL:
+			case Common::EVENT_RETURN_TO_LAUNCHER:
 				_shouldQuit = true;
 				break;