Creates a new row at @position. If parent is non-%NULL, then the row will be made a child of @parent. Otherwise, the row will be created at the toplevel. If @position is larger than the number of rows at that level, then the new row will be inserted to the end of the list. @iter will be changed to point to this new row. The row will be empty after this function is called. To fill in values, you need to call gtk_tree_store_set() or gtk_tree_store_set_value().