Provide auto-completion for properties of ApplicationWindow's contentItem

import QtQuick 2.3
import QtQuick.Controls 1.2

ApplicationWindow {
    id: window
    visible: true
    width: 200
    height: 200

    Rectangle {
        width: 50
        height: 50
        anchors.horizontalCenter: parent.horizontalCenter
        color: "transparent"
        border.color: "darkorange"
    }
}