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
STYLE: add tool tips to the clipping widget

Explain what is meant by the clipping terms and positive/negative
spaces.

Issue #2560



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22206 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: update clipping documentation

Use the text strings from the MRML clip node widget.

Issue #2560



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22207 3bd1e089-480b-0410-8dfb-8563597acbee