Allow callback function to be specified

Once the property has been validated, one may want to do some additional custom validation or trigger another action, so we could support a `callback=` argument that gets called with the class instance, the property name, and the property value after validation.

Locate the property-validation implementation and the existing tests for validated properties. Add coverage for the callback argument and verify that it receives the class instance, property name, and property value after validation.

Written by the indexing model from the issue text.