Add indicator for downloading a widget

[ccoelho]

While downloading a large csv/excel file (over several hundreds mb file) from the widget, there would be a long delay of couple minutes before the browser indicates the file has been downloaded. During this time, the user has no indication if the request to download is in process or has failed. It would be good to have an indicator to show the download of the file is in progress, so user would not click the download multiple times thus causing multiple queries to hit the backend.