Fix code relying on Project dock widget

[ptsavol]

This PR removes spine-items's dependency from Toolbox's Project dock widget.

Fixes spine-tools/Spine-Toolbox#2560

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

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

Comparison is base (38b05bd) 60.04% compared to head (08b0ece) 60.06%.

Files	Patch %	Lines
...ction/widgets/data_connection_properties_widget.py	0.00%	2 Missing :warning:
...ems/importer/widgets/importer_properties_widget.py	0.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #185 +/- ## =========================================== + Coverage 60.04% 60.06% +0.01% =========================================== Files 180 180 Lines 17697 17693 -4 Branches 2899 2899 =========================================== Hits 10627 10627 + Misses 6495 6491 -4 Partials 575 575 ```

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