Hide credentials from Importer's source URL widget

[soininen]

We now remove username and password from the source URL before showing it in Importer Specification editor.

Also, the source combo box has been replaced by a text field which makes more sense and simplifies code.

Resolves spine-tools/Spine-Toolbox#2429

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

Comparison is base (4577d95) 60.06% compared to head (1930ff7) 60.23%.

Files	Patch %	Lines
...ine_items/importer/widgets/import_editor_window.py	39.53%	48 Missing and 4 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #172 +/- ## =========================================== + Coverage 60.06% 60.23% +0.16% =========================================== Files 194 194 Lines 17615 17680 +65 Branches 2872 2887 +15 =========================================== + Hits 10581 10650 +69 + Misses 6468 6460 -8 - Partials 566 570 +4 ```

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