Displaying a previously created Matplotlib figure in a widget

まだ誰も着手していません。

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'`.

リンクされた Output Example Notebook をダウンロードして実行し、以前作成した figure、Output widget、HBox での動作を再現します。まず、表示された figure とその canvas がどのように処理されるかを追跡し、次に、figure が HBox 内で ipympl の対話性を維持していること、および fig1 を表示しても報告された NoneType エラーが発生しなくなったことを確認します。

索引モデルが issue の本文から書いたものです。