UITk MultiColumnListView Control is missing callbacks when being removed

A few issues we have in using the MultiColumnListView in hierarchy

• There is no Dispose function to control when to clear all elements
• Unbind is not called on the various cells when we close the HierarchyWindow
  • UnbindAll is not public on controller. I have a reflection hack for this.
• DestroyCell is NEVER called since the Column is always null in MultiColumnController.DestroyItem(VisualElement element)
• There is no way when quitting the HierarchyWindow to ensure DestroyCell is called on all active elements.
  • I tried ListView.itemsSource = null;ListView.Rebuild(); and it didn't work
  • the list of active items was empty when Rebuild got called.
  • When the active items pool is cleared, we don't call destroyItem