Port away from KIconThemes
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
Maniphest Tasks: T11637
Differential Revision: https://phabricator.kde.org/D26370