Help: Move resolution of help from multiple candidate IDs to HelpItem

No need for code duplication.

Change-Id: I3d2c795d072b8de5818e1844b8126e526339c0da
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/qmldesigner/designmodewidget.cpp

@@ -559,7 +559,7 @@ void DesignModeWidget::contextHelp(const Core::IContext::HelpCallback &callback)
     if (currentDesignDocument())
         currentDesignDocument()->contextHelp(callback);
     else
-        callback(QString());
+        callback({});
 }
 
 void DesignModeWidget::initialize()