Port away from KIconThemes

Summary:
IconSize is deprecated.

Using the "Desktop" icon size was somewhat arbitrary. There is no QStyle pixelmetric that offers the same size and fits semantically.

Test Plan: Side panel, properties dialog and extraction dialog look the same

Reviewers: Ark, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-utils-devel

Tags: Ark

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D26370