ENH: Changed label values in vtkMRMLSegmentationNode::GenerateMergedLabelmap

For label values generation use order of provided segment IDs instead of all segment IDs in the segmentation.
This is easier to use (as there is no need to look up the segment order in the original segmentation node)
and more flexible (order of label values can be chosen freely).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25457 3bd1e089-480b-0410-8dfb-8563597acbee