Remove irrelevant menu options for unsaved files

[kjswaruph]

Issue
Whenever a new library is created which is not saved yet displays The "Reveal in file explorer" and "Open terminal here" options which are uneccesary.
Changes: Disabled the "Reveal in file explorer" and "Open terminal here" options by removing factory.createMenuItem() for respective options

Mandatory checks

• 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.

[Siedlerchr]

But they need to be there for a saved library!

[HoussemNasri]

This does not fix the issue. First, we want to disable the relevant menu items not remove them. Second, the menu items should be kept enabled for saved libraries.

To tackle this, we need to figure a way to distinguish saved from unsaved libraries (hint: a saved library has a path in the filesystem). Then we need to bind (please read about JavaFX binding if you haven't already) that property to the enabled/disabled status of the show in folder and open console menu items.

[kjswaruph]

sorry for that i am now trying to only disable not remove the menu options.

[koppor]

Some code hints on disabling is available at #11941.

[Siedlerchr]

Superseded by #12199