Bring back the functionality needed to embed separate Bokeh roots within React

BokehJS 0.12.16 has embed/add_model_from_session function that can be used to embed an arbitrary root anywhere. In particular, I use it to embed separate components of the same document in different parts of a React application.

Commit 3eaa05a6fcef60abc7680d8b4465da0032750a35 removed this function, and I don’t think that right now there’s a way to embed separate roots without embedding the whole document at once.

Note that this comment may also be relevant: https://github.com/bokeh/bokeh/pull/7708#discussion_r235605875

[rsdenijs]

Just want to concur that the ability to dynamically embed new roots them at arbitrary locations on the page would be very helpful. It would allow the user to use bit and pieces of Bokeh in a non-bokeh layout. If there is a workaround for this I would love to know!

[bryevdv]

Ok I think adding a JS API to perform the same function as the template embedding (only explicitly and selectively, instead of automatically) is reasonably actionable. In fact it’s possible that such an API or something close already exists and we simply need to expose or document it better.

This is predicated on caring primarily about being able to control when/where individual roots are injected, but being insensitive to when/how the document is loaded (i.e. it should not be of explicit concern to users to control) Basically, sessions and documents should always be considered together (a session is simply the communication to support ongoing synchronizing an entire document)

I’ll try to write a small self-contained example that works on 0.12.16 sometime soon.

This is useful but I am also after a mockup/proposal of what would ideally liked to be had, that could evolve into an integration test

cc @mattpap