Refactor database names/DB editor window and tab management

[soininen]

This PR centralizes the management of database codenames (dropping the term 'codename') and moves the management of DB editor windows/tabs from DB manager to a dedicated machinery. This does not yet make the system perfect, but paves way for more sane DB editor tab naming and behavior. It fixes some current issues and problems, though.

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

Attention: Patch coverage is 75.08772% with 71 lines in your changes missing coverage. Please review.

Project coverage is 67.12%. Comparing base (18814fa) to head (26ab292).

Files with missing lines	Patch %	Lines
...x/spine_db_editor/widgets/multi_spine_db_editor.py	65.71%	3 Missing and 9 partials ⚠️
...toolbox/spine_db_editor/widgets/spine_db_editor.py	71.79%	10 Missing and 1 partial ⚠️
...ox/spine_db_editor/mvcmodels/pivot_table_models.py	50.00%	7 Missing and 1 partial ⚠️
...olbox/spine_db_editor/widgets/add_items_dialogs.py	55.55%	8 Missing ⚠️
...oolbox/spine_db_editor/widgets/custom_delegates.py	0.00%	6 Missing ⚠️
...toolbox/spine_db_editor/widgets/custom_qwidgets.py	40.00%	3 Missing ⚠️
..._db_editor/widgets/edit_or_remove_items_dialogs.py	72.72%	3 Missing ⚠️
...ox/spine_db_editor/widgets/manage_items_dialogs.py	0.00%	3 Missing ⚠️
spinetoolbox/database_display_names.py	95.83%	1 Missing and 1 partial ⚠️
spinetoolbox/plotting.py	33.33%	2 Missing ⚠️
... and 8 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2997      +/-   ##
==========================================
+ Coverage   66.99%   67.12%   +0.13%     
==========================================
  Files         190      193       +3     
  Lines       37084    37102      +18     
  Branches     4447     4445       -2     
==========================================
+ Hits        24845    24906      +61     
+ Misses      10978    10931      -47     
- Partials     1261     1265       +4     

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