Add overload for TransformMany to support IObservableList

[ronaldvdv]

• Added an extra constructor in TransformMany that accepts a Func<TParent, IObservableList<TChild>> as its manyselector. Since we want to be able to call the original manyselector again when the parent is removed (instead of remembering the value of manyselector(x).Items when the parent was added), I also added a small ManySelectorFunc class that captures the original selector.
• Added an overload of the TransformMany extension method too
• Duplicated and fixed the unit test for TransformMany to also test the new overloads

[ronaldvdv]

Solves #158

[RolandPheasant]

Thanks for this. I will leave the issue open as I will add some optimisations. Additionally I will add the same operator for the cache side of dd

[ronaldvdv]

Sounds good, thanks!