Eo composite: change composite objects to not be tied to parent

Eo composite: change composite objects to not be tied to parent

This commit breaks behaviour!
Re-parenting no longer detaches composite objects, so watch out.

Now you can have an object be a composite object of an object although
it's not its child. This allows widgets to do things like having an
object as the child of a child object while still making it a composite
object to the main object.

With this change, composite objects don't keep a reference to the child,
but instead composite "bonds" are implicitly removed when either the
parent or the child are destructed.