CreateDerivedCollection leaks subscriptions with no way to free them

[anaisbetts]

Calling CreateDerivedCollection will subscribe to the base collection as well as signalReset without any way to detach that subscription.

If you call this in an UpdateTheUI type of method (disregarding the fact that this is not very Functional™), old versions of the collection will be held in memory via the subscriber list of the base collection's CollectionChanged event.