Cleanup icon handling

Summary:
* Remove unnecessary KIconLoader search paths
* Prefer QStyle over KIconLoader for determining sizes
* Prefer icons over pixmaps on QActions

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D24801