Backupmanager: Include check for TimeStamp at .bak files

[koppor]

In case a .bak is older than a .bib - and it differs, a dialog is shown. This is not OK in a scenario, where git tooling, cloud synchronization or manual editing of the file was done. Thus, this PR adds checks for timestamps.

• Follow-up to Refine BackupManager #9054.
• Fixes Improve .bak detection koppor/jabref#597

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• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• 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.

[Siedlerchr]

Isn't the backup always older than the bib file?

[calixtus]

Not if onedrive, Dropbox, git or the user messes with the modification dates of the files.

[HoussemNasri]

I have the backupFileDiffers test failing locally. I logged the last modified time for the original and the backup files, and they are equal. After investigation, it turns out that Gradle was really fast and was able to compile both files at the same time, so both files were copied from the production resources folder to the build resources folder at the same exact time. Which led to the timestamp comparison returning 0 and eventually backupFileDiffers returning false.

After cleaning gradle cache and running the test again, this time it passed (For some reason, gradle was a little slower this time). I don't really have any fixes or suggestions. This seemed like the best place to document the problem in case it occur in production.