Nested pydantic model containers

[ndxmrb]

Hi @tlambert03,
when using nested pydantic models, I found this post on how to add a simple way of nesting widgets/containers. When trying to use the values, I needed to modify asdict() as well. Although there might be a reason you didn't just implement it already when answering the post (probably due to #317), I thought I'd make a pull request just in case.

Notably, some things are missing:

• Proper test e.g. for getting nested values from ValueWidgets and ContainerWidgets - as I didn't really know where to start
• QGroupBox option as mentioned in Improve nesting of containers with labels (group box) #317

Also, I tried my best with the types, but I probably don't oversee the grand scheme of things... 😅

I hope it's helpful, but I'm aware it might not be, so feel free to just close 😄

Thanks for having a look!

[ndxmrb]

I just saw that this probably messes with List types. Set to draft for now.

[codecov]

Codecov Report

Attention: Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.03%. Comparing base (379bc62) to head (15263c6).

Files with missing lines	Patch %	Lines
src/magicgui/schema/_ui_field.py	36.36%	7 Missing ⚠️
src/magicgui/widgets/bases/_container_widget.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #669      +/-   ##
==========================================
- Coverage   89.16%   89.03%   -0.14%     
==========================================
  Files          39       39              
  Lines        4746     4761      +15     
==========================================
+ Hits         4232     4239       +7     
- Misses        514      522       +8     

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