Fix Number of dimensions spin box in Importer specification editor

[soininen]

The lower limit in the spin box is now 0, not 1 allowing for 0-dimensional entities.

Fixes spine-tools/Spine-Toolbox#2422

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 (37f0ab2) 60.22% compared to head (a06820f) 60.23%. Report is 3 commits behind head on 0.8-dev.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #176 +/- ## ======================================== Coverage 60.22% 60.23% ======================================== Files 194 194 Lines 17678 17680 +2 Branches 2886 2886 ======================================== + Hits 10647 10649 +2 Misses 6462 6462 Partials 569 569 ```

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