Warn if filtering on mutable property without providing obvservable

```cs
var filtered = xs.AsReadOnlyFilteredView(x => x.P != null);
```

Where `P` is a mutable property.

Code fix can add:
```cs
var filtered = xs.AsReadOnlyFilteredView(
    x => x.P != null,
    x => x.ObserverPropertyChangedSlim(x => x.P));
```