Consider adding a "disableListMerging" feature to PropertyOptions

In some cases you might want not to merge values of a list property from different modules, but rather just set it in a product.

Use case is overriding the qbs.architectures property on a product, to set it to a single value.
Otherwise it might get merged with the default provided value, ending up with a list like ["x86", "x86"], which will cause NativeBinary.qbs aggregate property to be True, even though there's only one actual architecture to consider.

Discovered while trying to build a minumal ios-simulator application.