Add "make all paths relative" setting to project

[soininen]

This PR adds a project setting that controls whether external paths are stored as relative or absolute in project.json.

Re #3187

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 91.48936% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.82%. Comparing base (4e21418) to head (4105be2).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/project.py	40.00%	3 Missing ⚠️
spinetoolbox/helpers.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3191      +/-   ##
==========================================
- Coverage   68.96%   68.82%   -0.15%     
==========================================
  Files         198      198              
  Lines       38291    38303      +12     
  Branches     4586     4586              
==========================================
- Hits        26407    26361      -46     
- Misses      10568    10635      +67     
+ Partials     1316     1307       -9     

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