[Reg 5.15 -> 6.4.2] Loader with StackLayout and Repeater displays an incorrect index

When using `asynchronous: true` with a Loader that contains a StackLayout with a Repeater, the StackLayout assigns wrong indexes. This results in the wrong item being visible in the stack.

To reproduce the issue, please run the attached example project. 

1. Click on Load button. 0-0-0-1 is visible, this should be 0-0-0-0. The Repeater index should match the StackLayout index.
2. Use the ComboBox to select 3. Two items are now visible at the same time! The Repeater index 3 item's attached property StackLayout.index is still 0.