docs
(rasqal_literal_array_compare_by_order): Added sort+arg helper
(rasqal_variables_table_get_order): Added
rasqal_query_results_sort simplified

(rasqal_query_results_sort): Switch to use of internal helper for sorting
and rasqal_variables_table_get_order() to order the variables in a
result by name.

(rasqal_query_results_sort_compare_row): replaces rasqal_row_compare_arg()
and uses a private context struct rqr_context.

Update callers of rasqal_query_results_sort()