ipycanvas needs all referencing models to be pushed

[blois]

https://github.com/martinRenou/ipycanvas/issues/170#issuecomment-1119323393

Currently only models which are referenced by viewed widgets are instantiated in the sandboxed iframe. It sounds like similar to jsLink that ipycanvas has a controlling model that references the viewed widgets but is not referenced.

There should be a mechanism to pass all models that need to be instantiated when querying the state for a specific widget.

[maartenbreddels]

Might help to use this code to find 'connected' widgets:
https://github.com/jupyter-widgets/ipywidgets/pull/3114/files#diff-346aff842b126a18502cf33a55d587dbfe6c963c9991cf9f9bb56e205b88e41eR299

Which means, all widget that it references, or widget that reference the widgets required.