Force focus on current tab

[MaywarDeChamplain]

Closes #12931

JabRef sometimes closed the last-opened tab, not the currently visible (selected) tab.
Fixed by force focusing on the current tab when Ctrl+W is pressed

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• I own the copyright of the code submitted and I license it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (if change is visible to the user)
• 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.

[trag-bot]

The method getTabbedPane() returns a potentially null value. New public methods should not return null and should use java.util.Optional instead.

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[jabref-machine]

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[koppor]

Logic should compared with #12653