Allow registering xmlns prefixes for foreign assemblies

As pointed out in https://github.com/dotnet/wpf/issues/45, using

```cs
[assembly: XmlnsPrefix("http://schemas.dotMorten.com/2018", "custom")]
[assembly: XmlnsDefinition("http://schemas.dotMorten.com/2018", "MyClassNamespace")]
```

only works for assemblies you own.

It would be nice if you could declare those attributes at application level, to provide controls of an assembly with custom namespaces, if they lack those attributes, or even override them.

While the later one isn't essential, i think the first one could be useful, if people share their controls on nuget and don't know about custom namespace prefixes.

At least i've had situations where i was annoyed with the awfully verbose prefixes others chose, or the lack of them alltogether.

Another scenario would be that you are supplied with a viewmodels assembly and are going to use those in your views. Being able to group those namespaces would be neat.