Fix dnd reordering in linked files

[Siedlerchr]

Follow up from #12871

Refs #14587

Steps to test

1. Add a few linked files
2. drag over and drop one file over the other -> position is moved
3. Drag and drop an external file and see that it is added/warned if it exists

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.