Create a passive widget renderer and use as the default in PassiveCellRenderer

[stevejpurves]

Context

thebe has a PassiveCellRender class which can be used to render Jupyter outputs from json in web contexts where there is no kernel or no attempt to connect to one. That currently does not registrer a rendermime factory for ipywidgets where it could register a renderer that uses an offline WidgetManager (perhaps extending from HTMLWidgetManager) and that allows for injection of serialized widget state.

Proposal

Investigate how to add a "passive" WidgetManager as default rendermime renderer in PassiveCellRenderer. This should allow for model lookup, like the HTMLWidgetManager does, bu via some mechanism where widgets state in json can be supplied directly rather than expected in a script tag.

Tasks and updates

• Some initial experimentation is needed