Port from KDE::icon to KIconUtils for getting icon overlays

Summary: This also removes the last uses of KIconThemes from here.

Reviewers: apol, ngraham

Reviewed By: apol, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11654

Differential Revision: https://phabricator.kde.org/D26073