this function is broken. here's the right version (untested):
```julia
function selected(selection::GtkTreeSelection)
    model = mutable(Ptr{GtkTreeModelI})
    iter = mutable(GtkTreeIter)
    ret = ccall((:gtk_tree_selection_get_selected,libgtk),Cint,
          (Ptr{GObject},Ptr{Ptr{GtkTreeModelI}},Ptr{GtkTreeIter}),selection,model,iter)
    if !ret
        error("No selection of GtkTreeSelection")
    end
    convert(GtkTreeModelI, model[]), iter
```
However, note that this function already exists as `G_.selected`, the gtk_get_set_gen just needs to be taught about `GtkTreeIter`'s