* gtk/gtktreemodelsort.c: (gtk_tree_model_sort_row_changed):
        Consider reordering the row even when there's no sort column, if
        there's a default sort function.
        (gtk_tree_model_sort_rows_reordered): Use symbolic constants.
        (gtk_tree_model_sort_level_find_insert): Add code to handle the
        case where there is no sort column, but a default sort function.
        (gtk_tree_model_sort_insert_value): Add code to handle the case
        where there is no sort column an no default sort function.
        (gtk_tree_model_sort_set_model): Use symbolic constant.
        (gtk_tree_model_sort_reset_default_sort_func): Use symbolic constant.