Disable menu items for unsaved library

[x] I own the copyright of the code submitted and I licence it under the MIT license
[x] 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)
[x] 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.

JabRef / jabref