ENH: Added Transforms subject hierarchy plugin

Now the transforms are also displayed in subject hierarchy as nodes. Their tooltip shows relevant information, can be visualized via the regular show/hide buttons, and context menu includes Invert and Reset to identity features.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23790 3bd1e089-480b-0410-8dfb-8563597acbee