fix: fix splitting logic and deduplicate code in Groups Presets Widgets

[tlambert03]

fixes #363

This is just a patch for the immediate issue, these widgets still contain a lot of duplicated code and possible bugs, and should be replaced (with one of #307, #320, #338, etc...)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.69%. Comparing base (7b074d3) to head (9a7c04a).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   90.74%   90.69%   -0.05%     
==========================================
  Files          79       80       +1     
  Lines        9209     9160      -49     
==========================================
- Hits         8357     8308      -49     
  Misses        852      852              

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