Fix drag and dropping entries not always working

[HoussemNasri]

Fix #11254
Fix #11160

It doesn't work if the destination library has no group structure; "All entries" is the only group.

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.

[HoussemNasri]

Please don't squash-and-merge as this pull request fixes 2 issues.

[Siedlerchr]

codewise already looks good, will test it later

[Siedlerchr]

Tested, works fine! Thanks!

[HoussemNasri]

Awesome, do you think we can somehow add unit tests for the drag and drop logic? I'm out of ideas.

[Siedlerchr]

Hm, would probably need a GUI test for the drag and drop stuff...

[calixtus]

Unit tests for dnd would probably be much more effort. For now I think its enough to have it manually tested once.
As soon as the typos are fixed, squash is probably better anyway to avoid those typo commits in the main branch.
Fixing two closely related issues in one pr is not a problem imho.

[HoussemNasri]

@calixtus I think having the two commits separated is better because it leads to a cleaner history and would be easier to debug later in case of a regression. Plus it's possible to merge the typo fix commits into one of the exciting commits. It's very easy to do with IntelliJ.

[calixtus]

Go ahead.

[calixtus]

Looks good to me. High quality code, like always from you. Thanks!