Top-level item lists abandoned in tree models. Tree models each use a...

Top-level item lists abandoned in tree models.  Tree models each use a RootItem instead (all treeitems stored as internal pointers in model indexes now have non-NULL parents)

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2254 41f8697f-d340-4b68-9986-7bafba869bb8