ENH: Added labelmap node support to subject hierarchy

Subject hierarchy knew about the single vtkMRMLScalarVolumeNode class, but now that the vtkMRMLLabelMapVolumeNode has been introduced, a separate subject hierarchy plugin was created for labelmaps. This way it is cleaner that one role plugin assigns one icon and the same set of functionality to its owned nodes.

Fixed issue in sitkUtils: it created a scalar volume node regardless whether a labelmap was already created

Removed temporary contour references from model slice displayable manager (segmentations has its own displayable manager)

Added concept of virtual subject hierarchy branches, which is a branch for which its parent is solely reaponsible, i.e. the child nodes do not correspond to actual MRML nodes, but to items contained in the parent node to enable per-item handling from subject hierarchy (e.g. segments in segmentations and fiducials in markups)

Create and expose cloneSubjectHierarchyNode method in qSlicerSubjectHierarchyCloneNodePlugin to enable easy cloning of any node (clone SH node, data node, display and storage nodes)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24293 3bd1e089-480b-0410-8dfb-8563597acbee