Importer improvements support

[soininen]

Toolbox-side changes for a future PR in Spine-Items.

Re #3195

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 96.87500% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.03%. Comparing base (f1bdd75) to head (42eaad0).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...oolbox/project_item/specification_editor_window.py	96.29%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3214      +/-   ##
==========================================
+ Coverage   69.02%   69.03%   +0.01%     
==========================================
  Files         198      199       +1     
  Lines       38608    38625      +17     
  Branches     4633     4633              
==========================================
+ Hits        26649    26665      +16     
- Misses      10650    10651       +1     
  Partials     1309     1309              

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