Displaying a previously created Matplotlib figure in a widget

Personne n'a encore pris cette issue.

I'm trying to find a way to take a figure previously created, and show it inside a widget, _without losing interactivity._
Here's a short version of my approach (which may not be the right one):

```python
fig = plt.figure() # previously created figure
out = widgets.Output()
with out:
display(fig)
# other things happening
display(widgets.HBox([fig.canvas, some_other_fig]))
```

If I first create a figure in the notebook outside of an `Output` widget, and then wish to include it later inside something like an `HBox` next to a second figure, I seem to lose interactivity with the figure (including the nice ipympl zoom interface).

Here is a [nbviewer view](https://nbviewer.jupyter.org/github/thomasaarholt/tests/blob/master/Output%20Example%20Notebook.ipynb) of a MWE notebook I created, download it top right on the window. It's best if one downloads it and runs it since the first figure appears buggy in the nbviewer.

Additionally, and I might create a new issue for this - if I call `fig1` at the end of the notebook, I get an unexpected error. Here I'd expect the regular figure to be shown. The error is `AttributeError: 'NoneType' object has no attribute '_send_event'`.

Téléchargez et exécutez le Output Example Notebook lié afin de reproduire le comportement avec la figure créée précédemment, le Output widget et HBox. Commencez par retracer la manière dont la figure affichée et son canvas sont gérés, puis vérifiez que la figure conserve son interactivité ipympl dans HBox et que l’affichage de fig1 ne lève plus l’erreur NoneType signalée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.