feat: add QConfigGroupsModel, a QAbstractItemModel to manage configs and presets

[tlambert03]

pulling out from #446 ... this is the key model object that lets multiple views edit configuration groups and presets

[codecov]

Codecov Report

❌ Patch coverage is 94.05594% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.14%. Comparing base (f96a776) to head (3ddaa0f).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
...re_widgets/config_presets/_qmodel/_config_model.py	95.23%	13 Missing ⚠️
src/pymmcore_widgets/_icons.py	66.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
==========================================
+ Coverage   86.96%   87.14%   +0.18%     
==========================================
  Files          96       97       +1     
  Lines       10606    10891     +285     
==========================================
+ Hits         9223     9491     +268     
- Misses       1383     1400      +17     

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