Add love.dropbegan and love.dropcompleted callbacks.

Resolves #1346

src/modules/event/sdl/Event.cpp

@@ -428,6 +428,12 @@ Message *Event::convert(const SDL_Event &e)
 		}
 		SDL_free(e.drop.file);
 		break;
+	case SDL_DROPBEGIN:
+		msg = new Message("dropbegan");
+		break;
+	case SDL_DROPCOMPLETE:
+		msg = new Message("dropcompleted");
+		break;
 	case SDL_QUIT:
 	case SDL_APP_TERMINATING:
 		msg = new Message("quit");