UGGrid: append to-be-visited sons at the end of the list

Prepending them to the beginning of the list would mean they are never visited as the iterator only moves forward. This required switching to a std::list as the std::forward_list does not provide methods to add items at the end.

This regression was introduced in 20b821be which replaced Dune::SLList with std::forward_list, but changed push_back to push_front as well.

Closes: #53 (closed)