Add progress bar to the file input widget

[monodera]

As the web server is located at the observatory, it is expected that users need to wait a bit to finish the file upload. It's not an issue for a relatively small file, but maybe problematic for a large file. It would be nice to have a progress bar of the file upload.

Describe the solution you'd like
Add a progress bar for the file upload widget to indicate the progress of the transfer.

Describe alternatives you've considered
Just ask users to wait for a while before proceeding to the next steps.

Additional context
There is a possibility discussed in the following thread.
holoviz/panel#2863 (comment)

I think it's not quite sustainable, so I'd like to wait for being incorporated in Panel. (maybe someone should help to send a PR...)

[monodera]

Another reference: https://discourse.holoviz.org/t/working-file-input-with-progress-update/4665/1

[monodera]

I disable the buttons at start. Once the first file is uploaded, the "validate" button becomes active. 6e94085

[monodera]

The FileDropper widget is available in the master. I've not yet tested, but sounds promising.

holoviz/panel#2863 (comment)