Make Group(Node)TreeViewModel more OO

[koppor]

Follow-up to #9955

• Imperative style: X.doSomething(y) (static or pseudo-static method in X)
• OO style: y.doSomething()

I rewrote GroupNodeViewModel to be more OO.

More Java 20 would have been following code, but I would had to enable preview features which I did not dare:

    public boolean isEditable() {
        AbstractGroup group = groupNode.getGroup();
        switch (group) {
            case AllEntriesGroup g:
                return false;
            case AutomaticKeywordGroup g:
                return false;
            case AutomaticPersonsGroup g:
                return false;
            case TexGroup g:
                return false;
            case ExplicitGroup g:
                return true;
            case SearchGroup g:
                return true;
            default:
                assert group instanceof KeywordGroup;
                // KeywordGroup (which its children  LastNameGroup, RegexKeywordGroup, WordKeywordGroup)
                return groupNode.getParent()
                         .map(parent -> parent.getGroup())
                         .map(groupParent -> !(groupParent instanceof AutomaticKeywordGroup || groupParent instanceof AutomaticPersonsGroup))
                         .orElse(false);
        }
    }

Moreover, there is no chaining of classes (e.g., `case AllEntriesGroup | AutomaticKeywordGroup | AutomaticPersonsGroup g)

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.