Data connection marks invalid db refs red

[PiispaH]

Now when a db connection is not valid the reference is colored to red just as the file references are to indicate that there is a problem. By refreshing the connection can be checked again.

Fixes spine-tools/Spine-Toolbox#2375

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: 23 lines in your changes are missing coverage. Please review.

Comparison is base (063084b) 59.80% compared to head (808ace8) 59.83%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #167 +/- ## =========================================== + Coverage 59.80% 59.83% +0.03% =========================================== Files 193 193 Lines 17552 17583 +31 Branches 2848 2858 +10 =========================================== + Hits 10497 10521 +24 - Misses 6493 6502 +9 + Partials 562 560 -2 ``` | [Files](https://app.codecov.io/gh/spine-tools/spine-items/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [spine\_items/data\_store/data\_store.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvZGF0YV9zdG9yZS9kYXRhX3N0b3JlLnB5) | `67.74% <100.00%> (ø)` | | | [...pine\_items/data\_connection/widgets/custom\_menus.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvZGF0YV9jb25uZWN0aW9uL3dpZGdldHMvY3VzdG9tX21lbnVzLnB5) | `27.27% <0.00%> (ø)` | | | [spine\_items/database\_validation.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvZGF0YWJhc2VfdmFsaWRhdGlvbi5weQ==) | `61.53% <33.33%> (+5.12%)` | :arrow_up: | | [spine\_items/data\_connection/data\_connection.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvZGF0YV9jb25uZWN0aW9uL2RhdGFfY29ubmVjdGlvbi5weQ==) | `72.89% <60.86%> (-1.67%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/spine-tools/spine-items/pull/167/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools)

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