Add default hierarchical context in group creation

[aqurilla]

This fixes #9141 by persisting the most recently used hierarchical context within GroupPreferences. This saved context is used when opening the Add Group dialog the next time.

• 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]

Thank you for working on this.

You're definitely going on the right track.

However, I believe it would be great if I can do this without having to open the "JabRef Preferences" dialog. One option that comes to mind is to set the "Hierarchical context" to the option selected in the last session. In other words, if the user selected "Intersection," then closed and reopened the "Add Group" dialog, "Intersection" should be still selected.

You can keep most of the code you have written so far. The changes to GroupPreferences look right on point, so you shouldn't need to touch any of it. Just undo changes to GroupTab and use GroupPreferencesl#getDefaultHierarchicalContext to set the default hierarchical context in GroupDialogViewModel.

[aqurilla]

Thanks for your feedback @HoussemNasri. I've updated the PR with the requested changes - the GroupDialogViewModel now persists the most recently used hierarchical context within GroupPreferences

[HoussemNasri]

Can you please update the changelog entry? And maybe put it under Changed instead of Added.

[Siedlerchr]

Thank you very much! +1 also for adding the test cases!
Looking forward towards more contributions :)

[aqurilla]

Thanks, happy to help!