[poppler] Moving from qsort to std::sort

[Albert Astals Cid]

Yes, i know i've been a huge non std:: proponent in poppler for long time 
being a PITA for lots of people.

But these people where using lists or strings for which we have a "solution" 
that works for our use cases as well as the std ones.

Now qsort vs std::sort is a interesting story, i get up to 30% speedups 
(callgrind says) (in the function itself, obviously not in the total runtime) 
just exchanging qsort for std::sort.

So let's discuss if we want to allow std or not. And if we allow it, are we 
going all in? Or just allow some cases? But which?

Opinions?

Albert