State callbacks

State callbacks are added with the following methods:

• Validation().started()
• Validation().valid()
• Validation().invalid()
• Validation().changed()
• Validation().validated()
• Validation().error()

• Predicate().started()
• Predicate().valid()
• Predicate().invalid()
• Predicate().changed()
• Predicate().validated()
• Predicate().error()
• Predicate().restored()

It is a way to connect side effects to the state of a Validation or Predicate.