improve unlinked files dialog

[agondekova]

Description of changes:

I modified the display of file paths in the "File" column so that relative paths, starting from the directory specified in the "Directory" field, are shown instead of full absolute paths. I also added the ability to resize the columns in the table, making them customizable for the user. Additionally, I changed the alignment of the text in the column, which was previously aligned to the right, to the left, as described in the issue, to ensure proper display when the text doesn’t fit within the available space.

This change improves the clarity and flexibility when working with the table.

Closes #11878.

Mandatory checks

• [x] 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)
• [x] Tests created for changes (if applicable)
• [x] Manually tested changed features in running JabRef (always required)
• [x] 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.