Fix disable some actions when no libraries are open

[lzmmxh]

This PR improves the user interface by disabling certain menu items and toolbar buttons when no library is open.

"Save All" menu item in MainMenu.java is now disabled (greyed out) when there are no open libraries.
"New entry from plain text" menu item in MainMenu.java is now disabled when there are no open libraries.
"Global Search" bar in the main toolbar (MainToolBar.java) is now disabled when there are no open libraries.
"New entry from plain text" button in the main toolbar (MainToolBar.java) is now disabled when there are no open libraries.

Here's the screenshot:

Here's the demo video:

2024-10-12.19-15-50.mp4

Closes #11923

#11923

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.

[koppor]

Follow up PR: #11931

@lzmmxh Please keep the PR open and just push to the branch. No need to close a PR after a review.

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[koppor]

PR with the real fix: #11941