Fix calculation of large directory sizes

[soininen]

This fixes a bug in directory size aggregator where directory sizes larger than a 32bit int could not be displayed.

Fixes #3188

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.96%. Comparing base (a81d97e) to head (1d189ae).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/widgets/project_settings_dialog.py	50.00%	2 Missing ⚠️
spinetoolbox/widgets/settings_widget.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3190      +/-   ##
==========================================
- Coverage   68.97%   68.96%   -0.01%     
==========================================
  Files         198      198              
  Lines       38287    38291       +4     
  Branches     4584     4586       +2     
==========================================
  Hits        26407    26407              
- Misses      10564    10568       +4     
  Partials     1316     1316              

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