BUG: show the color indices rather than row number

The Color GUI was showing a row number starting from 1
when displaying color table entries, which are 0 indexed.
This led to a mismatch between the value shown in the
data probe (and in the actual color node) versus
what the user was being shown to be the color index.

Overloaded the headerData method to, in the case of the
vertical header display role to return the row index
minus 1. The MRML color model defines a ColorEntryRole
but it's still off by 1 from the color index.

Testing of copying a color node exposed the fact that the
new color node was not showing up in the Data module nor
in the Save Data dialog, so updated CopyNode in the color
logic to turn off the hide from editors flag on the new node.

Issue #1892


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22331 3bd1e089-480b-0410-8dfb-8563597acbee