ENH: Make subject hierarchy plugin method addNodeToSubjectHierarchy non-virtual

Now that subject hierarchy nodes are automatically created when a supported data node (i.e. there exists an SH plugin that can add it to the tree) is added to the scene, the qSlicerSubjectHierarchyAbstractPlugin::addNodeToSubjectHierarchy method lost its purpose, so although it's still used as a convenience function, it cannot be overridden any more.
Minor fixes: comments, checks, typos

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23816 3bd1e089-480b-0410-8dfb-8563597acbee