Correct mapFromSource method when reordering columns. fixes #20673

[troopa81]

Description

This PR corrects the issue https://issues.qgis.org/issues/20673
When we reorder column (last column becomes first column for instance) with select on top activated, and we select an entire row the currentIndex is no longer the first column, and consequently it breaks edition.

Checklist

• Commit messages are descriptive and explain the rationale for changes
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• New unit tests have been added for core changes
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[m-kuhn]

Thanks a lot.
Once the fix is confirmed, a backport will be needed.