feat: support nbagg matplotlib backend (WIP)

[maartenbreddels]

This would solve https://github.com/jupyter/nbconvert/issues/1081 and would also make the old legacy nbagg backend work with nbconvert and voila (at least render it).

However, I cannot get it to work, the backend does not send me back the image, which then should be injected in the div.

Note: if you read this, consider using ipympl instead https://github.com/matplotlib/ipympl

[maartenbreddels]

It now fails for the right reason, which is that the mime bundle only contains a div, while it should contain an image with the base64 encoded data. This is not implemented because I do not get the message containing the image data.

[maartenbreddels]

@tacaswell @martinRenou Maybe you have ideas on how to fix this? The protocol is a bit hard to digest.

[MSeal]

Thanks for opening the WIP