[WIP][GSOC22] - C - Improve the external changes resolver dialog

[HoussemNasri]

Contributes to #6190

• [ ] 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.