Set the ViewContainer name of qtModelEntityItemCombo to be fixed

The private view container in QComboBox was assigned an object name automatically if not named. This seems to be unreliable on different platforms which will cause inconsistent test failure on dashboard machines. For example, the ModelBuilderGroupOps test will pass on certain machines, but fail on others because this privateViewContainer seems to get different auto-generated name on some machine, thus cause unrecognized object while playing back the xml script. In this commit, we set the object name for the privateViewContainer so that there is no ambiguity on different machines.