Properly Copy Associated PDFs Between .bib Files

[rioxddd]

Describe the changes you have made here: what, why, ... This pull request fixes the issue where associated PDF files were not being copied when entries were moved from one .bib file to another. Now, when copying entries, the linked PDFs are also transferred to the correct folder for the target .bib file.

Changes Made: Updated copyEntry() to store the source .bib file context. Modified pasteEntry() to copy linked PDFs to the target .bib file's directory.

Link the issue that will be closed: (https://github.com/koppor/jabref/issues/520)

Mandatory checks

• [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 applicable)
• [ ] Tests created for changes (if applicable)
• [x] Manually tested changed features in running JabRef (always required)
• [ ] Screenshots added in PR description (for UI changes)
• [x] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [x] 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.

[koppor]

@rioxddd Do you need a review by a teammate here in this repository? If not, please submit a pull request to https://github.com/JabRef/jabref/pulls - because only PRs there go into the release.