Core: Return context help id by callback

...to support asynchronous providers.

Change-Id: I483489c74e7886d5bc2bf00b65540c3d2c7afee0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmldesigner/components/integration/designdocument.cpp

@@ -651,12 +651,12 @@ void DesignDocument::updateCurrentProject()
     viewManager().setNodeInstanceViewProject(currentProject);
 }
 
-QString DesignDocument::contextHelpId() const
+void DesignDocument::contextHelpId(const Core::IContext::HelpIdCallback &callback) const
 {
     if (view())
-        return view()->contextHelpId();
-
-    return QString();
+        view()->contextHelpId(callback);
+    else
+        callback(QString());
 }
 
 } // namespace QmlDesigner