Change qSort to std::sort

Summary:
qSort is depecreated in Qt5 in favor of std::sort.
In some workloads qSort can be more than 200 times slower than std::sort,
in many others qSort is a little bit faster.

Test Plan:
Select 50.000 small files and press Shift-Supr.
With qSort 11% of cpu

And times around 300.000 miliseconds.

Select the same 50.000 small files and press Shift-Supr, cancel and press Shift-Supr again.
With std::sort 3.3% of cpu

And times around 900 miliseconds.

Reviewers: Frameworks, dfaure, markg

Reviewed By: dfaure, markg

Subscribers: markg, apol

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10857