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
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• 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]

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 ⚠️
...tems/data_transformer/widgets/class_tree_widget.py	0.00%	1 Missing ⚠️
.../data_transformer/widgets/parameter_tree_widget.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             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.