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 #317

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

Thanks for having a look!

[codecov[bot]]

Codecov Report

:x: Patch coverage is 66.66667% with 9 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 89.13%. Comparing base (45a54d1) to head (5ab31e5). :warning: Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/magicgui/schema/_ui_field.py	57.89%	8 Missing :warning:
src/magicgui/widgets/bases/_container_widget.py	85.71%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #704      +/-   ##
==========================================
- Coverage   89.28%   89.13%   -0.16%     
==========================================
  Files          40       40              
  Lines        4816     4841      +25     
==========================================
+ Hits         4300     4315      +15     
- Misses        516      526      +10     

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