A Few Small Fixups

I added a few things to this great example, feel free to take them en-masse, or cherry pick the ones you like. The biggest one being that I made the WPF example use RxUI bindings instead of XAML bindings, which takes a lot of text cruft out of the XAML (making it more focused on actual visuals), as well as adding type-safety - if you rename a ViewModel property, the build no longer works.

This PR also takes advantage of ReactiveUI's Auto DataTemplate feature, where if you register an `IViewFor<ViewModel>` for an ItemsControl, you don't have to provide a data template. Feels a bit Overkill here, but it was a good example.