Fix opening infinite db editor tabs from same db

[PiispaH]

If a db_map is already opened in a db editor, new tabs with the same db_map won't be opened anymore. Instead the one already open is brought up. If however the same db_map is opened in a new window, it is allowed to open.

Re spine-tools/Spine-Toolbox#2435

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (44b6883) 77.07% compared to head (d10e09f) 76.93%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #337 +/- ## =========================================== - Coverage 77.07% 76.93% -0.15% =========================================== Files 74 68 -6 Lines 9205 9153 -52 Branches 1944 1622 -322 =========================================== - Hits 7095 7042 -53 Misses 1778 1778 - Partials 332 333 +1 ```

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