Implement drag and drop in Importer specification editor

[soininen]

It is now possible to

1. Reorder source tables
2. Reorder mappings
3. Copy mappings from one source table to another, even between different editor windows

by dragging and dropping.

Fixes spine-tools/Spine-Toolbox#2239

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 82.10863% with 56 lines in your changes are missing coverage. Please review.

Project coverage is 62.70%. Comparing base (b74d94b) to head (88e241d).

Files	Patch %	Lines
spine_items/importer/mvcmodels/mappings_model.py	76.47%	32 Missing and 20 partials :warning:
spine_items/importer/widgets/import_mappings.py	20.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #208 +/- ## ========================================== + Coverage 61.71% 62.70% +0.98% ========================================== Files 190 190 Lines 17697 17908 +211 Branches 2897 2952 +55 ========================================== + Hits 10922 11229 +307 + Misses 6191 6076 -115 - Partials 584 603 +19 ```

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