Provide "Cancel" option during Export

[philrz]

#2613 described a use case with an Export operation that might take several minutes to complete. At the moment the user is shown an "Exporting..." pop-up and has to just wait for it to complete. It would be helpful if there was a way to cancel the operation, such as via a button on the pop-up.

I'm not sure if these Export-related pop-ups are provided by the same library as the one mentioned in #2605, but if so, maybe this is something to cover at the same time as that issue.

In a wider team discussion, we also agreed that the "Exporting..." pop-up could also be improved with some kind of progress info streamed from the backend to let the user know how close it is to completing. When we add the option to cancel I can open a follow-on issue to track that improvement next.

[philrz]

As of Zui commit f1aef59 which includes the changes in #2985, the new workflow for export to a pool does have a cancel option, but export to a file still lacks the ability to cancel.