ENH: Subject hierarchy tree headers show icons instead of text

The text in the narrow columns ('Vis' and 'Tr') looked quite bad, so instead it shows the show/hide icon for the visualization column, and a transform icon in the transforms column. The columns are now not resized when exiting then entering the module.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23798 3bd1e089-480b-0410-8dfb-8563597acbee