ListView.section update context properties instead of required properties

If ListView is used as root in a custom component that requires additional properties this goes wrong.

The culprit is the following code for method getSectionItem in qquicklistview.cpp 

if (delegatePriv->hadTopLevelRequiredProperties()) {
    delegate->setInitialProperties(nobj, {{QLatin1String("section"), section}});
} else if (!reuseExistingContext) {
    context->setContextProperty(QLatin1String("section"), section);
}

This means that if required properties are used they are initialized. The context is not used.
Later when the section is updated with `setSectionHelper` we have a context and the lookup for the 'section' property always succeeds as the ListView is a parent that has a section property.
So the context property is updated but the required property of the delegate is left alone.

The following patch is needed:

const bool isBound = delegate->isBound();
auto delegatePriv = QQmlComponentPrivate::get(delegate);
const bool useRequiredProperties = delegatePriv->hadTopLevelRequiredProperties();
const bool reuseExistingContext = isBound || useRequiredProperties;

That way no context is initialized when the required properties are used anyways.
You can use useRequiredProperties in the conditions as well.