QmlDesigner: Fix material no longer downloadable if once canceled

Task-number: QDS-9618
Change-Id: Ic3bb6594611dc51e34e0a6e4456749539b1cf211
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

src/plugins/qmldesigner/utils/multifiledownloader.cpp

@@ -46,6 +46,7 @@ void MultiFileDownloader::setDownloader(FileDownloader *downloader)
 
 void MultiFileDownloader::start()
 {
+    m_canceled = false;
     emit downloadStarting();
 }