Weak Parent

[PureWeen]

Description of Change

iOS doesn't have the ability to garbage collect if there's a circular reference of NSObjects. In order for the GC to work, it's important for platform views to only reference down the tree and never up the tree.

This PR makes it so the Parent property on our xplat element is just using a WeakReference so that there isn't a strong reference from the children back up to the parents.

This shouldn't cause any issues with the Parent or Child being collected prematurely for a couple of reasons.

1. There should really never be a scenario where a child has parent set but the parent doesn't have the child added as a logical child.
2. If for some reason one isn't true, then, the parent should have a reference to the child at the platform level. For example, if you add a button to a layout, the underlying UIView will have a reference to the UIButton which means the UIButton won't get collected.

[PureWeen]

/rebase