Resume or retry stopped, interrupted, or failed uploads

if i'm uploading a large number of files, and i accidentally click away from the upload window or lose network connectivity, the upload's individual progress bars will just appear to freeze and not move, given the impression that it might still be uploading.

Here's what i think could be done to make the upload flow more clear and intuitive:

- add a button to explicitly cancel uploads
- add a confirmation when the user clicks outside of the upload popup (something like "uploads are still in progress, do you really want to cancel them?")
- detect when network connectivity is lost (iirc there is an `online` and `offline` event) and pop up an alert informing the user that they are offline and asking if they want to retry the uploads that failed
- if it's just a single upload that failed, let the user know (maybe show a semitransparent refresh icon over it to indicate that the user should click it to retry) and/or automatically retry until a retry limit is hit

Inizia con il popup di upload e le relative barre di avanzamento individuali, quindi esamina come gli eventi online/offline del browser siano correlati agli upload non riusciti. Il lavoro è completo quando gli utenti possono annullare deliberatamente gli upload, ricevere un avviso prima di uscire mentre sono presenti upload attivi e identificare e ritentare gli upload non riusciti dopo una perdita di connettività.

Scritto dal modello di indicizzazione a partire dal testo della issue.