ENH: add a utility method to add a new fiducial

Prevents memory leaks and error messages from adding a fiducial
and a display node. 

For access from python, use:
fidID = slicer.modules.annotations.logic().AddFiducial(10.0, -50.0, 8.8, 'P1')
fidNode = getNode(fidID)

Resolves issue #3138



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22205 3bd1e089-480b-0410-8dfb-8563597acbee