Added love.lowmemory event callback function, called when the app is close to running out of memory on iOS and Android.

--HG--
branch : minor

src/modules/event/sdl/Event.cpp

@@ -362,6 +362,9 @@ Message *Event::convert(const SDL_Event &e) const
 	case SDL_APP_TERMINATING:
 		msg = new Message("quit");
 		break;
+	case SDL_APP_LOWMEMORY:
+		msg = new Message("lowmemory");
+		break;
 	default:
 		break;
 	}