Warn if filtering on mutable property without providing obvservable

[JohanLarsson]

var filtered = xs.AsReadOnlyFilteredView(x => x.P != null);

Where P is a mutable property.

Code fix can add:

var filtered = xs.AsReadOnlyFilteredView(
    x => x.P != null,
    x => x.ObserverPropertyChangedSlim(x => x.P));