ENH: Subject hierarchy nodes automatically created for supported types

If a data node is added that is supported in subject hierarchy, it is automatically inserted in the tree. The notion of potential nodes completely removed. This reduces confusion about why certain nodes in the subject hierarchy tree don't have full functionality and icons. Generally, both code and usage became simpler.
Fixes #3891

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23784 3bd1e089-480b-0410-8dfb-8563597acbee