BUG: Show data node name in subject hierarchy

Previously, the node column of the subject hierarchy tree showed the name of the subject hierarchy node, stripped from the postfix. However, if the associated data node was renamed after the creation of the SH node, still the old name showed up in the tree. Observing Modified event and doing renaming would be an option, but it would trigger a lot of updates, potentially affecting performance. This commit solves this problem by showing the name of the associated data node if there is one.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24804 3bd1e089-480b-0410-8dfb-8563597acbee