UI Validation

[arqueror]

Currently we support UI validation by using 2 providers:

• FluentValidation (AbstractValidator type)
• ComponentModel.DataAnnotations.Validator (ObservableValidator, IValidatableObject and INotifyDataErrorInfo types)

We support individual property validation combined with real-time updates by adding an errors property for each model property:

This way we subscribe to the OnPropertyChanged event in the ViewModel to filter the validation errors based on the property name and assign and raise the appropiate error in UI:

For AbstractValidator instead of manually filtering the errors we only need to call the ValidateAsync() method in the ViewModel and then triggering the UI errors logic from a custom rule:

Finally in the XAML code we are just using a converter to show/hide an errors tooltip based on the related errors property content:

Full sample can be found in this PR: #1174