Port away from deprecated KIconLoader::IconSize

Summary: Use QStyle::pixelMetrics instead

Test Plan:
No compiler warning
Icon still happily bounces on application launch

Reviewers: KWin, davidedmundson, broulik

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D27726