BUG: Allow assigning procedural color nodes to map scalars

Previously the GUI assumed that color table nodes only were used to
map scalars to colors onto model geometry. This change generalises
the color node selector to allow both color table nodes as well
as procedural color nodes.
Renamed the widget and methods to reflect the generalisation.
Only set the scalar widgets if they don't match the model
display node settings.
Added a label displaying the scalar range.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22809 3bd1e089-480b-0410-8dfb-8563597acbee