QmlDesigner: Add flag to check for designer

The property runningInDesigner can be checked to find out
if a component is running in the designer.

Change-Id: Ibf35daba4bb30c7d919b1ff149c92b348d7e8059
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

share/qtcreator/qml/qmlpuppet/qml2puppet/instances/dummycontextobject.cpp

@@ -45,4 +45,9 @@ void DummyContextObject::setParentDummy(QObject *parentDummy)
     }
 }
 
+bool DummyContextObject::runningInDesigner() const
+{
+    return true;
+}
+
 } // namespace QmlDesigner