ENH: Subject hierarchy module added

This is the first step of integrating Subject hierarchy into Slicer core.
Next steps include moving subject hierarchy plugins to their related
modules and making DICOM import plugins add loaded objects in the
hierarchy.
More information: http://www.slicer.org/slicerWiki/index.php/Documentation/Labs/SubjectHierarchy

From: Csaba Pinter <csaba.pinter@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23235 3bd1e089-480b-0410-8dfb-8563597acbee