Expose TransparentForPositioner

Exposing TransparentForPositioner as a (attached?) R/W property of Item would be useful for multiple usecases.

One might want to instantiate a Item as a parent of another existing Item and position it relative to the existing Item.
This is not possible if the existing Item is in a positionner or layout, the container will take over the positionning.
I have encountered this usecase when trying to implement a reorderable delegate, a crossfading behavior and other cases I've since forgotten about.

A possible workaround is to embed the new Item in a 0 sized Item so that it gets ignored, but that is hacky, add to the memory footprint and is one more level of indirection.

Being able to set transparentForPositioner in user code would greatly simplify things and seems natural.