Determine icon sizes via QStyle, not via KIconThemes

Summary:
Allows us to get rid of the soon to be deprecated usage of the top-level
IconSize() function.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D25898