Improve line edits

[soininen]

This is the Spine-Items part of spine-tools/Spine-Toolbox#2633

Re spine-tools/Spine-Toolbox#2629

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 60.76555% with 164 lines in your changes are missing coverage. Please review.

Project coverage is 60.69%. Comparing base (569fa28) to head (69a4673). Report is 2 commits behind head on 0.8-dev.

Files	Patch %	Lines
spine_items/exporter/commands.py	20.33%	47 Missing :warning:
spine_items/commands.py	10.25%	35 Missing :warning:
spine_items/tool/commands.py	27.02%	27 Missing :warning:
spine_items/data_store/commands.py	52.63%	9 Missing :warning:
...e_items/tool/widgets/tool_spec_optional_widgets.py	76.31%	9 Missing :warning:
spine_items/view/commands.py	18.18%	9 Missing :warning:
spine_items/data_connection/commands.py	76.66%	7 Missing :warning:
spine_items/exporter/exporter.py	50.00%	5 Missing :warning:
spine_items/tool/tool.py	16.66%	5 Missing :warning:
...s/tool/widgets/tool_specification_editor_window.py	90.19%	4 Missing and 1 partial :warning:
... and 4 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #191 +/- ## =========================================== + Coverage 60.66% 60.69% +0.03% =========================================== Files 193 193 Lines 17687 17877 +190 Branches 2875 2891 +16 =========================================== + Hits 10729 10850 +121 - Misses 6375 6445 +70 + Partials 583 582 -1 ```

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