Bulk download doesn't show progress

In a bulk download scenario, a user will select one or more items and start a download. Feedback is given in the UI for the packaging process ("Preparing zip export", "Zip export is ready") but there is no on-screen indication of the download progress until the fetch completes and the named file appears in the browser's download manager.

The attached video shows the scenario. Note that nightly is being used in the video, and it is recorded from a browser in a location where the network is not too fast. At 0:04 you can see 'zip export is ready' then the fetch runs for 21.30s – then at 0:25 the downloaded item shows in the bottom bar.

Nuxeo Web UI should show the progress of the fetch. The solution should be an adequate example for Nuxeo API devs to follow (for example, for a client who uses their own custom UI that leverages Nuxeo APIs)