ValidatableModel: Add support for validating complex and collection properties

[jbe2277]

ValidatableModel uses System.ComponentModel.DataAnnotations for validation. Unfortunately, it does not support recursively validation of properties.

I encountered the situation where I have to validate an entire object graph and not only a single object (Application type: WPF and Xamarin Forms). I came up with a specific solution for my domain build upon the ValidatableModel class.

Idea: Provide a generic working solution for recursively validation of properties.

• Validate an entire object graph (consider cycles)
• Integrate the solution into ValidatableModel
• Compatible to System.ComponentModel.DataAnnotations
• Support for complex type properties and collections of complex types

Related GitHub .NET Issue with a proposal for such a solution

• Add support for validating complex or collection properties using System.ComponentModel.DataAnnotations.Validator