Validation system modification to include multiple contexts

[brignolff]

Hi everyone,

Had a case where i needed to have validations that stop and block an external process and others that simply display the error without blocking or stopping anything. I also needed to display these validations separately.

To solve that problem, I thinked of having multiple validation contexts in a single ViewModel.
To do so I made overloads for the "ValidationRule(...)" and "BindValidation(...)" methods. I added a "Func<TViewModel, ValidationContext> validationContextProperty" parameter to those methods and use it instead of the one present by default in the "IValidatableViewModel" interface.

I haven't figured out what to do with the default ValidationContext property as I prefer to use custom named ones. It might become a global context where all validations regroups, but validations would lose the context added by the custom name.