Add internal state validators for HasValue

[tsuoanttila]

This change is 

[elmot]

Reviewed 4 of 5 files at r1.
Review status: 4 of 5 files reviewed at latest revision, 2 unresolved discussions.

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server/src/main/java/com/vaadin/data/HasValue.java, line 301 at r1 (raw file):

    /**
     * Returns a validator that checks the internal state of the HasValue. This

Is the javadoc really clear for an app developer?
What about text like "This method should be overwritten for components with internal value validation like text input for dates. If user text is unparseableas a date, then that should be exposed to Binder by using default component validator".

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server/src/main/java/com/vaadin/data/HasValue.java, line 305 at r1 (raw file):

     * date. A faulty input from user can be seen as a validation failure.
     * 
     * @return internal state validator

@since

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Comments from Reviewable

[elmot]

Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[anasmi]

Still some problem exists with validation

• Set malformed date --> call a binder validation --> remove date field content-->call a binder validation another time => the validation failed. (Field is set to be in non-required mode)
  -Also setting malformed input (abc, for example) --> validating returns error, but no visual representation then setting the right value makes textfield red
  

Code to reproduce:
`

         final VerticalLayout layout = new VerticalLayout();

	DateField dateField = new DateField("Date");
	dateField.setParseErrorMessage("Parse error");
	dateField.setDateOutOfRangeMessage("Out of range");
	layout.addComponent(dateField);
	dateField.addValueChangeListener(event -> {
		System.out.println(dateField.getValue());
	});
	Label errorLabel = new Label();
	layout.addComponent(errorLabel);

	Binder<Void> binder = new Binder<>();
	binder.forField(dateField).withStatusLabel(errorLabel).bind(o -> dateField.getEmptyValue(), null);

	Button button = new Button("Validate!");
	button.addClickListener(event1 -> {
		if (binder.validate().isOk()) {
			System.out.println("Correct");
		} else {
			System.out.println(dateField.isEmpty() + "Error!");
		}
	});
	layout.addComponent(button);

	setContent(layout);`