ENH: add a new color node, continous around 0.0

Add a method to create a new procedural color node that's useful for
visualizing error statistics centered around 0, called RedGreenBlue.
Rename AddRandomNode to AddDefaultProceduralNodes and add the RandomIntegers as
well as the new RedGreenBlue node in it. This will provide a space to add
new default procedural color nodes.
Create a new color node category, Continuous, and place the new node
in it.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22811 3bd1e089-480b-0410-8dfb-8563597acbee