[WIP] Implement copying cross-references

[HoussemNasri]

Closes #6404

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.

[Siedlerchr]

Unfortunately, Localization.lang does not work with textblocks yet.... You need to use \n

[koppor]

I remember that I tried to switch to JavaParser or similar tooling for better localization handling. However, this did not work in all cases and lead to upstream issues.

I was thinking to use Kilt for JabRef. However, I did not see fast enough how to keep our localization checks. (Because I like that we have a check that the messages are verbatim included in the code). - And it seems, we would have to change all our parameterized strings (poiu-de/kilt#23). ( %0 to {0}). And some layer on top of Kilt (which is not that harmful, but effort. And thus, the effort added too much for me).

Refs #7279

[koppor]

Found the implementation: c6c7edd (#8050)

[Siedlerchr]

I think this ... should be part of the localization string. I would change the above one as well