VM: Enable custom native views that have child views #3704
Description
Some views (panels) support having children, so we need a new interface to add the following APIs to ABIViewManager:
- virtual void AddView(XamlView parent, XamlView child, int64_t index);
- virtual void RemoveAllChildren(XamlView parent);
- virtual void RemoveChildAt(XamlView parent, int64_t index);
- virtual void ReplaceChild(XamlView parent, XamlView oldChild, XamlView newChild);