Segment color refactor
Addresses the issue of segment color stored in multiple locations (vtkSegment::DefaultColor and vtkMRMLSegmentationDisplayNode::SegmentDisplayProperties::Color). It caused ambiguity and increased complexity when saving and loading. Also it was inconsistent with the recent changes related to terminologies, in that the current terminology information was stored in the segment, but the current color was not (DefaultColor was for keeping track of the original color from DICOM etc.).