Fix scenario filter for entity alternatives

[soininen]

This fixes a special case where an entity has entity alternative in an alternative that is not part of any scenario. Such entities are now properly filtered out when using the scenario filter.

Fixes #spine-tools/Spine-Toolbox#2504

Checklist before merging

• [ ] Documentation (also in Toolbox repo) is up-to-date
• [ ] 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 (b92facd) 77.39% compared to head (c33f91b) 77.39%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #352 +/- ## ======================================== Coverage 77.39% 77.39% ======================================== Files 75 75 Lines 9283 9283 Branches 1957 1957 ======================================== Hits 7185 7185 Misses 1773 1773 Partials 325 325 ```

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[manuelma]

Thank you very much for this @soininen