BUG: clean up the model node logic for toggling visibility.

Model slice nodes are distinguished by name rather than class, make that a more easily set string.
Don't toggle annotations or fiber bundle nodes or any other sub classes of 
model nodes.

Issue #2576



git-svn-id: http://svn.slicer.org/Slicer4/trunk@21687 3bd1e089-480b-0410-8dfb-8563597acbee