cancel method
Implementation
void cancel(DownloadItem item) async {
await service.cancel(item);
item.status = DownloadStatus.readyToDownload;
locallyQueuedIds.remove(item.id);
locallyDownloadingIds.remove(item.id);
running = (running - 1).clamp(0, maxParallel);
_processQueue();
_managePollingTimer();
downloads.refresh();
}