Multiple composable validation

[TysonMN]

Now that validation can be added as a composable effect, what should be the behavior when this effect is added more than once to a single binding? The current behavior is to ignore all but the last added validation effect. This is arbitrary and the code to do it is not elegant.

One different solution is to concatenate all the string lists obtained from all the validation effect into a single string list.

However, if we implement the suggestion in #362 to return errors to WPF using the type ObservableCollection<obj> and allow for keyed merges (i.e. merges using IDs), then it is not obvious how multiple validation effects could be merged.