Fix for issue 9717

[Toro520]

1. addCrossref(BibEntry crossrefEntry) method:

• Motivation: This method is used to add cross-references to the current entry.
• Description: This method takes a BibEntry object as a parameter and sets its citation key as the cross-reference field value of the current entry. This establishes a relationship between the entries. Then, the updateCell() method is called to update the relevant cell data.

2. addFileFromCrossref(LinkedFile file) method:

• Motivation: This method is used to add files from cross-referenced entries.
• Description: This method takes a LinkedFile object as a parameter and adds it to the file list of the current entry. Before adding, it checks if the cross-referenced entry already has the same file to avoid duplicates. If the file doesn't exist, it is added to the file list, and the updateFiles() method is called to update the files.

3. removeCrossref() method:

• Motivation: This method is used to remove cross-references.
• Description: This method removes the cross-reference field of the current entry. Then, the updateCell() method is called to update the relevant cell data.

4. updateCitationKey(String newCitationKey) method:

• Motivation: This method is used to update the citation key of cross-referenced entries.
• Description: This method takes a new citation key as a parameter and sets it as the citation key of the cross-referenced entry. Then, the updateCell() method is called to update the relevant cell data.

5. removeCrossrefedEntry(BibEntry crossrefedEntry) method:

• Motivation: This method is used to remove cross-referenced entries.
• Description: This method takes a BibEntry object as a parameter and calls its removeCrossref() method to remove its cross-reference field. Then, the updateCell() method is called to update the relevant cell data.

6. updateCell() method:

• Motivation: This method is used to update cell data.
• Description: This method contains the logic to update cell data. The specific implementation should be tailored to your code structure and requirements. You can add appropriate code in this method to update the relevant cell data.

7. updateFiles(List linkedFiles) method:

• Motivation: This method is used to update the file list.
• Description: This method takes a list of LinkedFile objects as a parameter and updates the file list data as needed. The specific implementation should be tailored to your code structure and requirements.

Fixes JabRef#7731
Fixes [https://github.com/JabRef/issues/7731]

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.

[Toro520]

First trying of issue #1 9717.