ENH: Add procedural color storage node and GUI support

Add a new storage node, register it, test it.
Added support for copying procedural nodes so that users can copy
an existing one and then save it to disk.
LoadColorFile expanded to work with both color table and procedural
color node loading.
TBD: add support for .cxml
Added support in the Colors gui to display and edit color transfer
functions (Continuous frame, with a ctkVTKScalarsToColorsWidget).
As well as allowing the scalar bar widget to be set from a procedural
color node.

Issue #3573



git-svn-id: http://svn.slicer.org/Slicer4/trunk@23084 3bd1e089-480b-0410-8dfb-8563597acbee