Web ui should show progress bar on file upload

When user is uploading a file via web interface, it is not really clear whether file is actually going to the server or it is stuck due to numerous possible hick-ups in proxy, network, cable, routers or server equipment.

I believe that upload process should create new widget that will display info about file upload process. The upload state should be sent via XMLHttpRequests and update the progress bar. Since Content-Length is sent by the client, it is just a matter of the server side writing file upload status information for the given client/given file.

If no confirmation about file being transferred is received in a timely fashion, the user might be notified that there is something wrong and advice him to retry the upload process.

This bug report is inspired by question #100387.