Update for ipywidgets 7.0 and JupyterLab 0.28

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Fixes #15.

• Removed export button and fixed the handling of interactivity toggling.
• Fixed the download image behavior since Chrome no longer supports data urls for window.open.

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@tacaswell @SylvainCorlay, I had one bug that I can't figure out. I sometimes get the following error that is cleared by restarting the kernel:

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cc @jasongrout

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Note that the video above shows it creating a new window to download. I changed it so that it automatically downloads '<figure_title>.png` without navigating away from the page.

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Note that the following also works (should this close #9)?:

%matplotlib ipympl
import matplotlib.pyplot as plt

plt.plot([0, 1, 2, 2])
plt.show()

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Found the source of the usage error:

https://github.com/ipython/ipython/blob/dfb458636b247a78f4a57206128caffd97035ea5/IPython/core/pylabtools.py#L27

Ensured that we always get "ipympl" in 676ea16.

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Ah, the problem is that the addition of widget in IPython.core.pylabtools needed a corresponding addition to https://github.com/ipython/ipykernel/blob/71c44fe80e79126de4f27e6713fda1d274132d77/ipykernel/eventloops.py#L45.

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PR: ipython/ipykernel#275

[SylvainCorlay]

Good catch for the IPython backend name!

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Sweet! Can you add me to the PyPI maintainers?

[SylvainCorlay]

Done!