kdalgorithms::remove_duplicates doesn't work if the type is not sortable

[narnaud]

Take a vector of QPoint, and use kdalgorithms::remove_duplicates with kdalgorithms::do_not_sort: I would expect this to do:
points.erase(std::unique(points.begin(), points.end()), points.end());

But it complains that the type has no operator<