Gtk::TreeIter: Make a real const_iterator

Rewrite the whole TreeIter class hierarchy, including TreeRow and
TreeNodeChildren. TreeRow and TreeNodeChildren (a TreeRow container) are no
longer subclasses of TreeIter. TreeIter is a template class, instantiated as
TreeIter<TreeRow> and TreeIter<const TreeRow> to get TreeNodeChildren::iterator
and TreeNodeChildren::const_iterator, a.k.a. TreeModel::iterator and
TreeModel::const_iterator. Bug #134520