Hide tab bar

[priyanshu16095]

Fixes #9971

This PR introduces a feature to hide the tab bar when only a single library is open in JabRef, enhancing the user interface for devices with low screen space.

THE CHANGES MADE

-When a single library is open, the tab bar is hidden.
-Ensured that the tab bar is restored when more than one library is open, maintaining expected functionality.

Video Link: https://drive.google.com/file/d/1Qg4dbx3hqh2hrzbcmbs-OculUrgtBdCf/view?usp=sharing

• I own the copyright of the code submitted and I licence 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 (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.

[github-actions]

Your code currently does not meet JabRef's code guidelines.
We use Checkstyle to identify issues.
Please carefully follow the setup guide for the codestyle.
Afterwards, please run checkstyle locally and fix the issues.

In case of issues with the import order, double check that you activated Auto Import.
You can trigger fixing imports by pressing Ctrl+Alt+O to trigger Optimize Imports.

[priyanshu16095]

I am facing some problems with this PR, as it is also showing the changes made in the previous PR. Additionally, the checks are failing for the changes from the previous PR, even though the previous PR passed all checks.
It does not include all the commits of the previous PR.

[github-actions]

Your code currently does not meet JabRef's code guidelines.
We use Checkstyle to identify issues.
Please carefully follow the setup guide for the codestyle.
Afterwards, please run checkstyle locally and fix the issues.

In case of issues with the import order, double check that you activated Auto Import.
You can trigger fixing imports by pressing Ctrl+Alt+O to trigger Optimize Imports.

[priyanshu16095]

Please review it and provide any suggestions.

[Siedlerchr]

When you create a new branch, you first need to switch to main otherwise the branch will be created at the commit of the other branch

[priyanshu16095]

Yes, I did that, but still made this mistake.
I will fix it.

[calixtus]

Great start
I have two suggestions:

• Do not directly overwrite the css-Style with set style but create a class that should be set or removed. Otherwise user made changes to the style will be overwritten.
• Add a preference option to enable/disable this behaviour.

Also I have a question that should be talked about:

• right now each tab has a right-click context menu with some quick links, that are otherwise included in the menu. Should there be another place for the context menu to open?
• What about the close button? Should there be a new x button the on the top right of the maintable, if the close button in the tab is not shown?