Disable some actions when no libraries are open

[LoayGhreeb]

When no libraries are open in JabRef, some actions remain active but should be disabled, as they depend on an open library to function.

Actions to disable:

• File → Save all
  
• Library → New entry from plain text
  
• Global search button
  
• New article button
  

[lzmmxh]

Hey, I am new and would like to make an attempt and contribute to this issue.

[github-actions]

Welcome to the vibrant world of open-source development with JabRef!

Newcomers, we're excited to have you on board. Start by exploring our Contributing guidelines, and don't forget to check out our workspace setup guidelines to get started smoothly.

In case you encounter failing tests during development, please check our developer FAQs!

Having any questions or issues? Feel free to ask here on GitHub. Need help setting up your local workspace? Join the conversation on JabRef's Gitter chat. And don't hesitate to open a (draft) pull request early on to show the direction it is heading towards. This way, you will receive valuable feedback.

Happy coding! 🚀