LayoutProxyItem should parent items to its parent, not to itself

`LayoutProxyItem` is a new addition that can be useful to make responsive layouts. But it's usefulness is severely limited by how it works. It currently seems to reparent it's target to itself when it becomes visible. That messes with layouts, and reduces the ways it can be used.

For instance, if you use a `Repeater` as a target, it doesn't work as expected. Normally, you'd use a repeater inside the context of a positioner or a layout, but because LayoutProxyItem makes the item a child of itself, there is no way to have a layout then layout the items created by the repeater.

A useful feature that LayoutProxyItem currently has is that it allows you to set Layout attached properties on either the item itself, or on the LayoutProxyItem. That is useful, and it would be great if that could be kept.