Item Count in a group updated correctly

[samiyac]

Fixes issue #4679

This PR fixes the first issue. I could not reproduce the second issue. The item count should now be updated correctly.

The problem was because of the Listener not getting notified when changes were being made. This was due to the weak listener which was attached to ObservableList returned by databaseContext.getDatabase().getEntries().
https://stackoverflow.com/questions/44341400/whats-the-purpose-of-fxcollections-unmodifiableobservablelist

The only case where this PR might not work is when we are dealing with a library in which there are no groups other than the All Entries #4441. That is because the group being utilized in the line text.textProperty().bind(group.getHits().asString()); in file /gui/groups/GroupTreeView.java is the group All Entries that is present even before you open any library in JabRef. This causes value to not be set properly. But the only time it happens is if the only group you have in the new library is All Entries. Once we are able to fix that, this PR should work in all cases.

---

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[tobiasdiez]

Thanks for the investigation and the fix!
I really don't like the weak listeners. They always bite me ;-).

Is this the only instance where listeners are registered against the list returned by getEntries? I would also be nice if you could add a small comment in the code explaining why we need to hold a reference to the list (because some IDEs would suggest to convert the field to variable with field scope).

[samiyac]

Is this the only instance where listeners are registered against the list returned by getEntries? I would also be nice if you could add a small comment in the code explaining why we need to hold a reference to the list (because some IDEs would suggest to convert the field to variable with field scope).

I am not sure about that. I can check for other instances over this week.