ENH: Added modified event for SetNthMarkupAssociatedNodeID

Rational: Each time a markup is placed in 3d, we want to identify
the object it is placed on. This information is stored as associated
Node id. When a new mark up is placed, associated node id is set to
blank (" ") by default , MarkupAddedEvent is triggered and then associated
node id is set to appropriate node id. So there is no way of identifying
the associated node id of the newly added node using events.

This commit updates the function "SetNthMarkupAssociatedNodeID" so that
the event "NthMarkupModifiedEvent" is invoked.

Reviewed-by: Nicole Aucoin <nicole@bwh.harvard.edu>
Reviewed-by: Steve Pieper <pieper@bwh.harvard.edu>

From: Sumedha Singla <sumedha.singla@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24921 3bd1e089-480b-0410-8dfb-8563597acbee