Displaying a previously created Matplotlib figure in a widget

Nobody has claimed this yet.

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'`.

Download and run the linked Output Example Notebook to reproduce the behavior with the previously created figure, Output widget, and HBox. Start by tracing how the displayed figure and its canvas are handled, then verify that the figure retains ipympl interactivity in the HBox and that displaying fig1 no longer raises the reported NoneType error.

Written by the indexing model from the issue text.