Fix misplaced Checkbox in Prefs/PreviewTab

[calixtus]

Fixes bug in #5451
Should be pretty self-explanatory.

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• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[calixtus]

Is it intended, that the preview still shows up in the requiredFieldsTab?

[Siedlerchr]

I thought the same. It should be an either or

[tobiasdiez]

That's happening if you make last-minute changes without double checking these changes. Thanks!

And no, the entry preview should show either as a tab or in split pane. For me that worked.