Add ipywidgets backend

[tlambert03]

This PR will add ipywidgets as a widget backend for the jupyter environment (using magicgui.use_app('ipynb')). Things are coming together pretty well, with most widgets and events working nicely:

https://user-images.githubusercontent.com/1609449/103779235-76028c80-5001-11eb-8504-20907c620c62.mov

[jni]

😍

[tlambert03]

Hey @Czaki, if you have a moment to review this, mostly paying attention to the minimal changes outside of the new backends/_ipynb directory (i.e. the stuff that has any potential to interact with existing non-notebook usage) that would be great.

Since it's a new/independent feature that doesn't really affect much of the Qt-based usage, I'll probably merge this soon to make it available for playing with

[codecov[bot]]

Codecov Report

Merging #87 (288e0d0) into main (284999e) will decrease coverage by 1.68%. The diff coverage is 64.15%.

:exclamation: Current head 288e0d0 differs from pull request most recent head 080e366. Consider uploading reports for the commit 080e366 to get more accurate results

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   89.02%   87.33%   -1.69%     
==========================================
  Files          30       33       +3     
  Lines        3953     4208     +255     
==========================================
+ Hits         3519     3675     +156     
- Misses        434      533      +99     

Impacted Files	Coverage Δ
magicgui/widgets/_bases/widget.py	84.92% <26.66%> (-2.98%)	:arrow_down:
magicgui/backends/_ipynb/widgets.py	63.91% <63.91%> (ø)
magicgui/backends/_ipynb/application.py	90.90% <90.90%> (ø)
magicgui/backends/__init__.py	100.00% <100.00%> (ø)
magicgui/backends/_ipynb/__init__.py	100.00% <100.00%> (ø)
magicgui/backends/_qtpy/widgets.py	86.95% <100.00%> (ø)
magicgui/widgets/__init__.py	100.00% <100.00%> (ø)
magicgui/widgets/_bases/button_widget.py	100.00% <100.00%> (ø)
magicgui/widgets/_protocols.py	100.00% <100.00%> (ø)
magicgui/_type_wrapper.py	65.70% <0.00%> (-2.53%)	:arrow_down:
... and 1 more

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[tlambert03]

thanks @Czaki ... i made all the changes requested