Layout Templates and matplotlib figures

Nobody has claimed this yet.

see #117 (issue) and #174 (PR) for some discussion on the topic.

@martinRenou I have been playing with using [Layout Templates](https://ipywidgets.readthedocs.io/en/latest/examples/Layout%20Templates.html) to stack figures in fun ways. Unfortunately, since matplotlib is currently responsible for the figure size, I end up with situations such as the following, where the figures are all cut off a bit. This is particularly noticable in `jupyter notebook` since the screen width is reduced.

In the following example, I stack a 4x4 of buttons and then a 4x4 of figures. The buttons stretch nicely, but the figures do not.

```python
%matplotlib widget
import matplotlib.pyplot as plt
from ipywidgets import TwoByTwoLayout
from ipywidgets.widgets import Button, Layout

# Buttons
def button(name):
return Button(description = name, layout=Layout(height='auto', width='auto'))

a,b,c,d = button("1"), button("2"), button("3"), button("4"),
layout_2x2 = TwoByTwoLayout(top_left=a,
top_right=b,
bottom_left=c,
bottom_right=d)
display(layout_2x2)

# Figures
figs = []
plt.ioff()
for i in range(4):
fig, ax = plt.subplots()
ax.imshow([[1,2], [3,4]])
figs.append(fig.canvas)

a,b,c,d = figs

layout_2x2 = TwoByTwoLayout(top_left=a,
top_right=b,
bottom_left=c,
bottom_right=d)
display(layout_2x2)
```

In `jupyter lab` the problem is less so, but still unfortunate that figures basically ignore the flexbox layout the widgets are designed with. I would set the size with matplotlib, but since I don't know the size of the viewport the user is using, it's likely that the size I choose will be too small or too large depending on the user.

I don't have a concrete solution at hand, but I'm wondering if we can come up with some logic that preserves aspect ratio but otherwise fills a widget to its fullest extent?

Start by reproducing the Layout Templates example in the issue in Jupyter Notebook and JupyterLab, then read the discussion in #117 and the existing work in #174. The desired outcome is not yet defined; clarify an approach that lets matplotlib figures fill the widget layout while preserving their aspect ratio, then establish how the behavior should be verified.

Written by the indexing model from the issue text.