Make Importer's Select all check box more intuitive

[soininen]

'Select all' can now be unchecked even when there is no source tables. Also, new tables created in sourceless modes are born as checked/unchecked depending on 'Select all'.

Fixes spine-tools/Spine-Toolbox#2627

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 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 60.84%. Comparing base (52dacaf) to head (da41881).

Files	Patch %	Lines
spine_items/importer/mvcmodels/mappings_model.py	90.90%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #192 +/- ## =========================================== + Coverage 60.74% 60.84% +0.09% =========================================== Files 193 193 Lines 17878 17879 +1 Branches 2891 2892 +1 =========================================== + Hits 10860 10878 +18 + Misses 6436 6415 -21 - Partials 582 586 +4 ```

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