Removed "NodeViewModel.OldValue" property

Removed ```NodeViewModel.OldValue``` property in preparation for moving ```EngineController.GetMirror``` into the scheduler world. We will start throwing exceptions in the following query APIs if they are not called from the context of ```ISchedulerThread```:

 - ```EngineController.GetMirror```
 - ```EngineController.GetGraphicItems```
 - ```EngineController.GetStringValue```

Hi @aparajit-pratap, please take a look, thanks! :)