Fix the preview panel showing the wrong entry (an entry that is not selected in the entry table)

[HoussemNasri]

Fix #9172

Bug Summary

In PreviewViewer, both setEntry and setLayout will assign the entry and layout fields respectively and call the update method to reflect the new UI state. The thing is that update would start a background task to update the UI. The setLayout method would run the update UI background task on the previous entry because the entry field assignment happens in setEntry. While setEntry would run the task with the new entry. The bug happens when the task started by the setLayout method finishes last which will result in the overriding of the display of the new entry preview made by setEntry.

Mandatory checks

• 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]

Thanks for digging into it and fixing it! Really great