Assert that IFormValidator instance is added to the form

This enhancement is related to this (my) invalid bug https://issues.apache.org/jira/browse/WICKET-5336

Documentation for IFormValidator states:

"... These validators are added to the form..."

It would be fine if there is check in FormComponent (add(final IValidator<? super T> validator) method), that IValidator is NOT IFormValidator, something like

Args.isFalse(validator instance IFormValidator, "validator");

Another place where check should be added is overriden bind(Component component) method of AbstractFormValidator

public void bind(Component component)

{ Args.isTrue(component instance Form, "validator"); }