ENH: Associated hierarchy node with ChildNode(Added/Removed)Event

It will now be easier to know which node has been added or removed
from a given hierarchy.

Still need to figure out which node to associated with the event
'ChildNodeRemovedEvent' invoked in function 'vtkMRMLHierarchyNode::RemoveHierarchyChildrenNodes'

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22910 3bd1e089-480b-0410-8dfb-8563597acbee