Provide access to default QIconEngine implementation

The internal QPixmapIconEngine implementation in qicon.cpp is a useful default, but modifying only one aspect of it requires a complete reimplementation of QIconEngine, as QPixmapIconEngine is not accessible either as a class nor as an instance.

Getting access to the QPixmapIconEngine instance used would make it possible to at least forward function calls from a QIconEngine implementation, but ideally the class would be public.