Fix duplicate database mappings when opening, adding or creating databases in DB editor

[soininen]

This fixes a bug in DB editor where same databases in different editor tabs did not get the same updates.

No associated issue.

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 55.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.20%. Comparing base (403c082) to head (3f9da4b).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/spine_db_manager.py	37.50%	5 Missing ⚠️
...x/spine_db_editor/widgets/multi_spine_db_editor.py	75.00%	1 Missing and 1 partial ⚠️
...toolbox/spine_db_editor/widgets/spine_db_editor.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3250      +/-   ##
==========================================
- Coverage   70.21%   70.20%   -0.02%     
==========================================
  Files         204      204              
  Lines       39210    39222      +12     
  Branches     4728     4731       +3     
==========================================
+ Hits        27533    27537       +4     
- Misses      10380    10386       +6     
- Partials     1297     1299       +2     

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