Fix GroupTreeView not updating when generating groups dynamically

[HoussemNasri]

Fixes #7129

Video Demonstration :

• Change in CHANGELOG.md described (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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[tobiasdiez]

Thanks for working on this!
Your current solution sadly doesn't scale to huge databases with a lot of groups. Some users have 10k entries with many groups, and creating the groups tree in such a situation takes some time. Since you are essentially recreating the whole groups tree on every field change, this would sadly freeze JabRef (ok at least make typing very very sluggish).

Since the AutomaticGroup already gets the list of entries, it can listen for changes on the entries in principle without the need to reconstruct the whole groups tree. However, I was not able to make that work (but also didn't tried to hard):

jabref/src/main/java/org/jabref/model/groups/AutomaticGroup.java

Lines 27 to 32 in f22968e

	public ObservableList<GroupTreeNode> createSubgroups(ObservableList<BibEntry> entries) {
	// TODO: Propagate changes to entry list (however: there is no flatMap and collect as TransformationList)
	return entries.stream()
	.flatMap(entry -> createSubgroups(entry).stream())
	.collect(TreeCollector.mergeIntoTree(GroupTreeNode::isSameGroupAs));
	}

[HoussemNasri]

Thanks for your fast feedback.
I didn't think about scale, the problem turns out to be harder than I thought. I am closing this since it can't be merged