Replace ProjectItemModel related stuff

[ptsavol]

This PR removes ProjectItemModel dependency from spine-items

Fixes spine-tools/Spine-Toolbox#2560

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

[soininen]

Do we use the item categories for anything after ProjectItemModel is gone?

[ptsavol]

Do we use the item categories for anything after this change?

No, category.py is gone.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (b86ccec) 60.18% compared to head (f080448) 60.66%.

Files	Patch %	Lines
...ne_items/data_transformer/specification_factory.py	0.00%	1 Missing :warning:
...tems/data_transformer/widgets/class_tree_widget.py	0.00%	1 Missing :warning:
.../data_transformer/widgets/parameter_tree_widget.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #187 +/- ## =========================================== + Coverage 60.18% 60.66% +0.48% =========================================== Files 194 193 -1 Lines 17789 17686 -103 Branches 2902 2875 -27 =========================================== + Hits 10706 10729 +23 + Misses 6509 6374 -135 - Partials 574 583 +9 ```

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[soininen]

No, category.py is gone.

That is so nice! We could then drop the category stuff from ItemInfo as well if that was not included in these PRs already.