QtConcurrent: use nth_element to calculate the (correct) median

Sorting is O(NlogN) complexity, while nth_element is linear.
Also remove the errornous +1 when calculating the median position.

Change-Id: Ib39085b59a6c5d15a3a940b1ce3377080340bc09
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>