BUG: copy opacity when creating a copy of a color node

The MRML color logic was using the the color node copy 
method that just copied the name and rgb, switched it 
to use the one that also copies opacity. 
Expanded the testing to exercise the copy and test the 
results.

Issue #3114


git-svn-id: http://svn.slicer.org/Slicer4/trunk@21988 3bd1e089-480b-0410-8dfb-8563597acbee