Fix Attribute Table header labels when adding multiple columns

[CodeBardian]

Retrieving the column labels in the attribute table from the source model addresses some cases where the attribute table would not show a column name when adding more fields.

This was intended to fix #55533. I just noticed it seems to unintentionally also fix #49142.

Fixes #55533
Fixes #49142

[github-actions]

🪟 Windows builds ready!

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(Built from commit 068e83e)

[github-actions]

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[nyalldawson]

Thanks @CodeBardian !