Get icon size from QStyle instead of KIconLoader

Summary: The sizes from KIconLoader are considered to go away in KF6

Test Plan: Open archive. Icons in file view are properly sized

Reviewers: Ark, elvisangelaccio

Reviewed By: Ark, elvisangelaccio

Subscribers: broulik, kde-utils-devel

Tags: Ark

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D25378