Gtk::TreeRow and TreeNodeChildren: Make real const versions

Split TreeRow and TreeNodeChildren into TreeRow+TreeConstRow and
TreeNodeChildren+TreeNodeConstChildren. Since TreeRow and TreeNodeChildren
are just disguised iterators, they too shall have const versions that can't
be implicitly copied to non-const versions that can be used for changing the
underlying TreeModel.
Add const versions of TreeModel::get_iter(). Bug #134520