BUG: set LUT range widget to trigger setting range on LUT

Updating the GUI element will trigger a signal to update the look up table
on the color node.
Renamed a method from setNumberOfColors to updateNumberOfColors as it wasn't
taking an argument.
TODO: set up an observation on the look up table on the color node

Issue #3115



git-svn-id: http://svn.slicer.org/Slicer4/trunk@21999 3bd1e089-480b-0410-8dfb-8563597acbee