StackLayout: Add attached property 'index'

TabBar {
  id: tabBar
  TabButton {
    text: "one"
  }
  TabButton {
    text: "two"
  }
}
StackLayout {
  currentIndex: tabBar.currentIndex
  Loader {
    source: "One.qml"
    active: parent.currentIndex === StackLayout.index // <- to be added
  }
  Loader {
    source: "Two.qml"
    active: parent.currentIndex === StackLayout.index // <- to be added
  }
}