clear items in vtkCellPicker’s list before destructing

We have to clear actors in vtkCellPicker object before InitializePickList.

In the function Pick, the picker object will addItem to increase its reference count.

So we need to do the following tasks for picker object in class destructor function to avoid crash or weird visible bug.

m_ItemPicker->GetActors()->RemoveAllItems();
m_ItemPicker->GetProp3Ds()->RemoveAllItems();
m_ItemPicker->InitializePickList();
m_ItemPicker->RemoveAllObservers();