Revisit "Allow signals forwarding in QML"

I propose revisiting https://bugreports.qt.io/browse/QTBUG-9291

1. It's received more interest over the years
2. Since then, `ui.qml` has been introduced. In a ui.qml file, you even do simple forwarding like button.onClicked: root.clicked()–this triggers a warning and I believe it stops QDS. You must add a whole connections block to forward your signal.   

Example situation: forward a signal in a .ui.qml file

Current (4 lines of code, 6 physical lines of code, 1 new block):

MyComplexComponent {
signal leftButtonClicked(QtObject mouseEvent)
 .........
          MyDeeplyNestedButton  {
             id: leftButton           
           }

  Connections {
    target: leftButton
    function onClicked (mouseEvent)  {       
      leftButtonClicked(mouseEvent);     
    }
  }
}

Desired (1 line of code, no new blocks):

MyComplexComponent  {
  signal alias leftButtonClicked: leftButton.clicked
 .........
          MyDeeplyNestedButton {
             id: leftButton           
          }
}

And of course this shortcut is useful in standard qml, not just ui.qml!