Implements selection client in catalog detail view for product variants

Important: The order product ID is now the ID of the parent product of a selection (not of the article any more) while the product code is still the code of the article

Additionally, fixes a problem in the basket controller with incomplete and non-unique variant attributes when checking selection products
Extends ProductPrice plug-in to handle selection products correctly
Adapts the unit test data and the dependent tests in the catalog index manager tests