Resizing extends outside the bounds of the notebook

Nobody has claimed this yet.

## Describe the issue

If you drag the resize handle outside the limits of the notebook and release the mouse you lose the ability to resize as the handle will be hidden:

This is a consequence of using a window mouse move listener: https://github.com/matplotlib/ipympl/blob/6b44a6f90cf74b12cfcf930b440010984d8a9b4d/js/src/mpl_widget.js#L272

Maybe there could be a check that it won't be resized to be larger the layout (`this.model.get('layout')`)?
## Versions

```
3.7.6 (default, Jan 8 2020, 19:59:22)
[GCC 7.3.0]
ipympl version: 0.5.6
jupyter core : 4.6.1
jupyter-notebook : 6.0.3
qtconsole : 4.6.0
ipython : 7.12.0
ipykernel : 5.1.4
jupyter client : 5.3.4
jupyter lab : 2.1.5
nbconvert : 5.6.1
ipywidgets : 7.5.1
nbformat : 5.0.4
traitlets : 4.3.3
Known nbextensions:
config dir: /home/ian/anaconda3/etc/jupyter/nbconfig
notebook section
jupyter-matplotlib/extension enabled
- Validating: OK
nbdime/index enabled
- Validating: OK
voila/extension enabled
- Validating: OK
jupyter-js-widgets/extension enabled
- Validating: OK
JupyterLab v2.1.5
Known labextensions:
app dir: /home/ian/anaconda3/share/jupyter/lab
@axlair/jupyterlab_vim v0.12.2 enabled OK
@jupyter-widgets/jupyterlab-manager v2.0.0 enabled OK
@jupyter-widgets/jupyterlab-sidecar v0.5.0 enabled OK
@jupyterlab/git v0.20.0-rc.0 enabled OK
jupyter-matplotlib v0.7.2 enabled OK
jupyterlab_vim-system-clipboard-support v0.1.0 enabled OK
nbdime-jupyterlab v2.0.0 enabled OK
```

Read js/src/mpl_widget.js around line 272 and reproduce the resize-handle behavior by dragging beyond the notebook bounds. Inspect how the window mousemove listener uses this.model.get('layout'); the issue is done when releasing outside the notebook does not leave the handle hidden or prevent further resizing.

Written by the indexing model from the issue text.