Nested containers

[ndxmrb]

Hi @tlambert03,
restarting my work from #669 after the ValuedContainer changes.

Still, some things are missing:

• Proper test e.g. for getting nested values from ValueWidgets and ContainerWidgets. Does this belong to test_pydantic()in test_ui_field.py?
• QGroupBox option as mentioned in Improve nesting of containers with labels (group box) #317

Tried my best with the types again, hope it's okay that way.

Thanks for having a look!

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.15%. Comparing base (45a54d1) to head (0b88814).

Files with missing lines	Patch %	Lines
src/magicgui/schema/_ui_field.py	41.66%	7 Missing ⚠️
src/magicgui/widgets/bases/_container_widget.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #704      +/-   ##
==========================================
- Coverage   89.28%   89.15%   -0.14%     
==========================================
  Files          40       40              
  Lines        4816     4831      +15     
==========================================
+ Hits         4300     4307       +7     
- Misses        516      524       +8     

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