Scene and sibling references handling modified.

- references are set to null it referenced component is not available
- references are set to null when object is removed from scene/parent
Property parentTransform changed to get/set.
TransformableSkin nested transform changes.

- if available, TransformableSkin uses sibling Transform2D
- if now available, it uses its internal transform
- both transforms handle parent transforms properly
clone() methods patched (probably not properly implemented).

Needs testing and probably few changes.