Fix issues with remote (non-Spine) databases

[soininen]

• Data connection's database URL validation was broken for SQLite databases and tried to connect to remote databases without credentials
• Importer did not properly recognize database URLs as valid data source when double-clicking the icon

Fixes spine-tools/Spine-Toolbox#2329

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

Comparison is base (61b23e2) 55.05% compared to head (95b74eb) 55.08%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #168 +/- ## ========================================== + Coverage 55.05% 55.08% +0.03% ========================================== Files 191 191 Lines 17378 17381 +3 Branches 2848 2848 ========================================== + Hits 9567 9575 +8 + Misses 7315 7311 -4 + Partials 496 495 -1 ``` | [Files](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?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\_connection/data\_connection.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvZGF0YV9jb25uZWN0aW9uL2RhdGFfY29ubmVjdGlvbi5weQ==) | `74.75% <100.00%> (ø)` | | | [spine\_items/models.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvbW9kZWxzLnB5) | `65.48% <0.00%> (ø)` | | | [spine\_items/database\_validation.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvZGF0YWJhc2VfdmFsaWRhdGlvbi5weQ==) | `64.47% <50.00%> (+4.19%)` | :arrow_up: | | [spine\_items/importer/importer\_factory.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvaW1wb3J0ZXIvaW1wb3J0ZXJfZmFjdG9yeS5weQ==) | `86.84% <0.00%> (ø)` | | | [spine\_items/importer/importer.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvaW1wb3J0ZXIvaW1wb3J0ZXIucHk=) | `51.80% <0.00%> (ø)` | | | [...ine\_items/importer/widgets/import\_editor\_window.py](https://app.codecov.io/gh/spine-tools/spine-items/pull/168?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVfaXRlbXMvaW1wb3J0ZXIvd2lkZ2V0cy9pbXBvcnRfZWRpdG9yX3dpbmRvdy5weQ==) | `46.52% <20.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/spine-tools/spine-items/pull/168/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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