Fix#19129

[lzmmxh]

This PR improves the user interface by disabling certain menu items and toolbar buttons when no library is open.
Specifically:
"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 #19129

#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.

[lzmmxh]

Modify changelog.md and fix some format problem.

[github-actions]

You modified Markdown (*.md) files and did not meet JabRef's rules for consistently formatted Markdown files.
To ensure consistent styling, we have markdown-lint in place.
Markdown lint's rules help to keep our Markdown files consistent within this repository and consistent with the Markdown files outside here.

You can check the detailed error output by navigating to your pull request, selecting the tab "Checks", section "Tests" (on the left), subsection "Markdown".