Add support for entity alternatives into Importer

[soininen]

With this PR, it is possible to import entity alternatives using Importer.

Resolves spine-tools/Spine-Toolbox#2748

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[bot]]

Codecov Report

Attention: Patch coverage is 78.28571% with 38 lines in your changes missing coverage. Please review.

Project coverage is 62.38%. Comparing base (93e1bce) to head (e8aa5f9).

Files	Patch %	Lines
spine_items/importer/mvcmodels/mappings_model.py	14.28%	12 Missing :warning:
spine_items/importer/flattened_mappings.py	81.13%	6 Missing and 4 partials :warning:
spine_items/importer/commands.py	30.76%	9 Missing :warning:
...e_items/importer/widgets/import_mapping_options.py	50.00%	6 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #215 +/- ## ========================================== + Coverage 62.33% 62.38% +0.04% ========================================== Files 168 168 Lines 17750 17832 +82 Branches 2955 2979 +24 ========================================== + Hits 11064 11124 +60 - Misses 6080 6103 +23 + Partials 606 605 -1 ```

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