External files for widget-state and/or binary data / with progressbar

[maartenbreddels]

Following up on the discussion here:

• Added the possibility to store the data in an external file. widgetti/ipyvolume#21
• Embedding with buffers #1282

It makes sense to maybe attack both of these after 7.0.

The idea is that once the widget-state gets huge, you want to have some of that data loaded async, like the whole widget-state, or the binary data in it, and then show a spinner of progress bar as indicator of the download.
This will make pages load faster, and give an idea what is going on.