ENH: Added support in vtkMRMLNode for multiple references to the same node

Fixed limitations of node references:

• Node reference modification request (SetAndObserveNthNodeReferenceID)
  was ignored if only events were different. This caused an issue in
  OpenIGTLinkIF, because different nodes in the same role required
  different event observations and so the role-default events that were
  restored from the scene were not correct.

• One node could only be referenced a single time only, otherwise the
  most recently added/removed/modified reference overwrote all the
  previous event observations. This limitation was not acceptable,
  because even a temporarily created reference (the user selects a node
  as input that is already selected as an input in another node selector)
  could remove event observations. This caused an issue in Fiducial
  registration wizard, which has two input fiducial lists, and when the
  user selected one input fiducial list it often removed the observer
  from the other input fiducial list.

Other related changes:

• Simplified node references implementation: all reference
  add/remove/modification operations go through a single method
  (SetAndObserveNthNodeReferenceID), previously there were many potential
  execution path for these operations, which made it difficult to make
  changes consistently.

• Fixed ObserverManager to update the observation even if only events or
  priorities are changed.

• Made vtkMRMLNodeReference data members ReferencingNode, ReferencedNode,
  Events accessible only by get/set functions to allow consistency checks.

• Renamed vtkMRMLNode::RemoveAllNodeReferenceIDs to RemoveNodeReferenceIDs
  to be consistent with similar method names

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24121 3bd1e089-480b-0410-8dfb-8563597acbee