Closes #13382,

The underlying issue was that the GroupTreeViewModel listens to changes in stateManager.activeDatabaseProperty() and clears the groups only when it becomes empty. However, when the last tab was closed manually (via CloseDatabaseAction), the stateManager was not notified that there were no more open databases, so the groups were never cleared.

I initially tried addressing this from inside GroupTreeViewModel:

 private void onSelectedGroupChanged(ObservableList<GroupNodeViewModel> newValue) {
        if (!currentDatabase.equals(stateManager.activeDatabaseProperty().getValue()) && currentDatabase.isPresent()) {
            // Switch of database occurred -> do nothing
            return;
        } else if (currentDatabase.isEmpty()) {
            // All databases being closed -> clear groups
            rootGroup.setValue(null);
        }

This initially worked after also clearing groups in the closeAllDatabaseAction, but would not work with the single close database action.

To have the single action having the same behavior, I did this and it worked also for the close all action:

        if (tabbedPane.getTabs().isEmpty()) {
            stateManager.setActiveDatabase(null);
        } 

This seems to fix the bug for all the actions (so probably the statemanager was never notified when there was not active tabs), checking there's no tabs open and updating the active database triggering all the UI updates.

Feedback needed.
Also for testing purpose I did a little button in the recent tab menu to open all the recent tabs (visible in the video) , let me know if it's would be a good adding to JabRef and I will push this too.

Steps to test

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (if change is visible to the user)
• 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.