Rework LogicalChildren on Element to better represent visual vs logical children

[PureWeen]

Description

Copied From xamarin/Xamarin.Forms#6106

This PR
#9221

Started the idea at the shell level for generalizing the AddLogical / RemoveLogical calls. Ideally, we'd move this up to Element or tie it naturally into parenting. For example, if I set someone as my parent then I automatically become a logical child. It's currently very difficult for someone to opt into our LogicalChidlren flow which also has negative effects on LVT and forces users to implement IVT which only makes it so a control will take advantage of the Live Visual Tree but then won't take part in BC and Style propagation.

The other advantage here is that it will allow control developers an easy hook point to implement the same behavior. They can attach logical elements to pages or elements and our Framework will auto propagate css, binding, etc..

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[PureWeen]

#16046