Don't propogate events for null children
Improve VariadicTemplates to use RX.CombineLatest

The locking code in WhenAny looks a bit risky. CombineLatest
from RX does the same thing.

With a few changes in ReactiveNotifyPropertyChangedMixin we can
reuse the code that we allready have and just punt the observables
to CombineLatest

Just have two tests failing now.

ItemsControlShouldGetADataTemplate
OFPShouldWorkWithINPCObjectsToo