Follow-up to #343 - Embedding Static Plots

Nadie ha tomado este issue todavía.

#343 was a huge step forward for saving the state of plots in notebooks. But it doesn't truly reproduce the state of the notebook that the author was looking at for these two cases:

1. If widget state is saved then only a blank figure renders (https://github.com/matplotlib/ipympl/pull/343#issuecomment-920167467)
2. Any updates to the figure after the initial render are not saved in the notebook (https://github.com/matplotlib/ipympl/pull/343#issuecomment-921008926)

I think part 2 is very important to be able to tell a story using a notebook. e.g. If I make some plots with a slider, get them just so and then run nbconvert I don't think I would faithfully reproduce the state of the notebook as I was looking at it.

**Part 1** should be solvable if we include the PNG as part of the widget model

**Part 2** I think a reasonable tradeoff would be to only update the static plot whenever the notebook is saved. This would require the frontend of ipywidgets (jupyterlab-manager, and widgetsnbextension) to listen to notebook save signals and pass them to the ipympl frontend, which would in turn let the backend know to update the mimebundle.

@martinRenou how hard do you think the second one (saving) would be to fix? To me that feels like a very important thing to hit.

Empieza leyendo cómo se gestionan las señales de guardado del notebook en jupyterlab-manager y widgetsnbextension, y después sigue el recorrido del frontend al backend de ipympl para actualizar el mimebundle. Comprueba el manejo del modelo del widget para el caso PNG. Se considera terminado cuando el estado guardado del widget se renderiza en lugar de mostrar una figura en blanco y las actualizaciones posteriores al renderizado se conservan al guardar el notebook y en nbconvert.

Escrito por el modelo de indexación a partir del texto del issue.