QQmlComponent can't use JS types for initial properties

QQmlComponent::createWithInitialProperties and setInitialProperties take a QVariantMap of properties and don't 'unbox' QVariant(QJSValue), so it's impossible to set JS types as initial properties.

This makes some types with required properties impossible to (correctly) create from the C++ API. It also means Qt.binding() can't be used like it can with Component.createObject() or Loader.setSource(). See the attached testcase.