AppMan: Let IntentModel and ApplicationModel support multiple layers of filtering/sorting

IntentModel {
    id: im1
    sourceModel: IntentServer
    sortFunction: function(li, ri) { return li.name < ri.name }
}

IntentModel {
    id: im2
    sourceModel: im1
}

Repeater {
    model: im2
    ...
}

int IntentModel::indexOfIntent(Intent *intent)
{
    int idx = IntentServer::instance()->indexOfIntent(intent);
    return idx != -1 ? mapFromSource(idx) : idx;
}

int IntentModel::indexOfIntent(Intent *intent)
{
    int idx = -1;
    if (auto src = qobject_cast<IntentServer*>(sourceModel()) // Most likely
        idx = src->indexOfIntent(intent);
    else if (auto src = qobject_cast<IntentModel*>(sourceModel()) // Less likely
        idx = src->indexOfIntent(intent);
    return idx != -1 ? mapFromSource(idx) : idx;
}