WIP: Add browser rendered previews/snapshots of html and widgets

[maartenbreddels]

This adds a preprocessor that spins up a webserver, points the browser to it, and make it render jupyter-widgets or html output to a png, which is added to the 'mime bundle'. This allows for instance the rendering of ipyleaflet to pdf, and will be an important ingredient for sphinx-gallery supporting widgets, see sphinx-gallery/sphinx-gallery#422
Example of ipyleaflet rendered:

TODO

• support for headless chrome with cmd line argument tweaking
• unittests

[maartenbreddels]

Nice side effect, it renders on github also (when both mime types are included, widget and png):
https://gist.github.com/maartenbreddels/5c143f66281d7d0e5866bf705ed6ce45

[piorek]

Hi, @maartenbreddels any updates on this?

[maartenbreddels]

Not really, I think some of this should be pushed to jupyter-widgets/ipywidgets#2280
But I also wonder if #1031 is the right solution?
Any strong opinions?

[bollwyvl]

Haven't had a chance to move forward on the qt-based stuff roughly demoed on #1031 <#1031>, but I do see it is as the most robust technique I've tried thus far for full documents with vector graphics. I should definitely try it out with lab 1.0!