Update sample to use FsXaml instead of FSharpX Type Provider

The old FSharpX type provider is no longer maintained.

This pull request updates your samples to use the new FsXaml Type Provider which has replaced it.  This is far more useful and functional, and supports many things the old provider didn't (such as using nested user controls, etc).

Also, the MVVM sample application was updated to use FSharp.ViewModule and be "fully" MVVM (no "code behind" style logic at all now).  This demonstrates slightly better practices, as it's keeping the logic encapsulated in an ICommand within the ViewModel.