BUG: add a way to set the slice intersection thickness

Add the SliceIntersectionThickeness property to vtkMRMLDisplayNode and test it.
Add a label and spin box to the qMRMLDisplayNodeWidget but keep it disabled for now.

Issue #2267



git-svn-id: http://svn.slicer.org/Slicer4/trunk@21832 3bd1e089-480b-0410-8dfb-8563597acbee