Port away from KIconThemes
Summary:
This was only used here for the about to be deprecated IconSize() method,
and some pixmap code that is better served by QIcon directly.
Reviewers: mlaurent
Reviewed By: mlaurent
Subscribers: kde-pim
Tags: KDE PIM
Maniphest Tasks: T11637
Differential Revision: https://phabricator.kde.org/D25957