Speedup sort
Summary:
Use a lambda function instead of a class.
This way the member QCollator is not copied and initialized several times.
Test Plan:
Sorting in a directory with 82874 images:
[TIME] Sorting: 19883 (before)
[TIME] Sorting: 4198 (after)
kfileitemmodelbenchmark before: .............. Passed 29.36 sec
kfileitemmodelbenchmark after: .............. Passed 20.39 sec
Reviewers: Dolphin, Frameworks, markg, elvisangelaccio
Reviewed By: Dolphin, markg, elvisangelaccio
Subscribers: elvisangelaccio, apol, bruns, markg, kfm-devel
Tags: Dolphin
Differential Revision: https://phabricator.kde.org/D13814