COMP: code clean up to unify setting the active place node

Added a utility function in the logic to set the active place node,
matching the get active place node id method. Call it where the
selection node was being accessed, and make sure that the testing
and setting of the ActivePlaceNodeClassName and ActivePlaceNodeID
are done only when there is a change and with the Reference call
so that events are triggered.
Added a test of the new SetActiveListID method.



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22483 3bd1e089-480b-0410-8dfb-8563597acbee