Add Debug to BackupManager

[koppor]

To debug #9488, this adds debugging.

This PR builds on #9678 - will be adapted as soon as #9678 is merged.

Opening the PR here to get a binary which can be tested.

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Reading #9488 (comment), this is currently not needed any more. @ThiloteE If you encounter this issue again, please have the latest .bak file and the .bib file placed somewhere else. - We can also craft another debug version again.