ENH: Implement feature #2918, fiducial projection on 2D slice plane

From: Laurent Chauvin <lchauvin@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21882 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: avoid print out of nodes not found on start up

Took out the call to update the widget from mrml on init, 
it gets called from setMRMLScene which is when it should be
updated.
Swapped warning for debug print outs as not having interaction
or selection nodes is not an error case.



git-svn-id: http://svn.slicer.org/Slicer4/trunk@21883 3bd1e089-480b-0410-8dfb-8563597acbee