BUG: Fixed minor issues in subject hierarchy parse local data plugin

Show/hide icons didn't show up for non-volume displayables; If the user did a parsing, loaded further data and did a new parsing, the first batch appeared twice in the list. Both issues have been fixed.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23726 3bd1e089-480b-0410-8dfb-8563597acbee