QDeclarativePropertyMap should have a virtual function to override write operations

It should be possible to create subclasses of QDeclarativePropertyMap so that write behaviour can be overridden.

The specific use case is to allow the assignment of single object to be placed in a list (with only one element)
e.g
foo.bar = obj results in foo.bar being the same as [obj]
but
foo.bar = [obj1,obj2,obj3] results in foo.bar being [obj1,obj2,obj3]