QmlDesigner: Add missing emit

Change-Id: I81bb36f586dd596c03bfe0fe8fefa543d6333397
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>

src/plugins/qmldesigner/components/toolbar/toolbarbackend.cpp

@@ -549,6 +549,7 @@ void ToolBarBackend::updateDocumentModel()
         m_openDocuments.append(entry->displayName());
 
     emit openDocumentsChanged();
+    emit documentIndexChanged();
 }
 
 int ToolBarBackend::documentIndex() const