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