Add a QIconEnginePlugin to allow QIcon deserialization

Summary:
This is necessary to allow QIcons with a KIconEngine as engine to deserialize
properly.

BUG: 399989

Test Plan: Ran the PoC in the bug report, works fine.

Reviewers: #frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: lbeltrame, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D16305