Binding events directly to an external bindable will cause that
bindable to hold a permanent reference to the current object.
We use `GetBoundCopy()` or otherwise a local bindable + `.BindTo()` to create a
weak-referenced copy of the target bindable. When the local bindable's
lifetime expires, so does the external bindable's reference to it.
This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
