Add asynchronous loading to Repeater

If there are hundreds complex delegates it takes time for Repeater to instantiate them. While the items are being created the user interface is blocked.
I suggest adding asynchronous loading and this property to Repeater:
asynchronous : bool
Currently items can be created asynchronously by either using Loader or Instantiator.
However

• Loader sets itself as the item parent an breaks the layout chain as it can't behave as Layout item.
• For each item a separate Loader component has to be created which is rather heavy and slow when it comes to hundred items.
• Instantiator doesn't work with layout items such as ColumnLayout or Column. There are no singals when item order is changed. Parenting/reparenting the items manually doesn't work fast enough an prone to glitches (layout items don't always get recalculated when new elements are added via parenting)

As Instantiator in its current implementation is basically useless I suggest merging it with Repeater