Fix some check boxes in Exporter, Importer

[soininen]

Inspired by spine-tools/Spine-Toolbox#2581

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 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 60.66%. Comparing base (d52ed10) to head (7d235b5).

Files	Patch %	Lines
...tems/importer/widgets/multi_checkable_list_view.py	0.00%	3 Missing :warning:
spine_items/exporter/widgets/preview_updater.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #189 +/- ## =========================================== - Coverage 60.66% 60.66% -0.01% =========================================== Files 193 193 Lines 17686 17687 +1 Branches 2875 2875 =========================================== Hits 10729 10729 - Misses 6374 6375 +1 Partials 583 583 ```

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