BUG: Re-enable scrollbar for Subject hierarchy tree view

It's better to have a double scrollbar than being confused about the layout (if there are many nodes in the tree, then a scrollbar is added to both the module widget and the tree view), so I re-add the scrollbar temporarily until a better fix is made.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23722 3bd1e089-480b-0410-8dfb-8563597acbee