Access the Locator via RxApp.DependencyResolver

This PR adds a property (`RxApp.DependencyResolver`) back that just returns the Locator from Splat, but also causes ReactiveUI to initialize itself.

This way we don't need an `EnsureInitialized` method anymore, which you can easily forget to add.

The name of the property is debateable. Maybe `RxApp.Locator` is a better name?