Add an AggregateListModel

[jrbarron]

This model can aggregate several other models using QML defaults property feature so it will look something like this:

AggregateListModel {

    JsonListModel {
        id: model1
    }

    JsonListModel {
        id: model2
    }

    JsonListModel {
        id: model3
    }
}

We might need to add some attached function property to the JsonListModels that can transform the returned JSON in case they are different for each model.