Provide API to opt-out of on-blur validation for input elements

[Shridhad]

Currently the validation for input elements triggers on blur event. It would be great to have a way to opt-out of this automatic validation.

<vaadin-text-field required 
          error-message="Please enter value" 
          value="{{value}}">
</vaadin-text-field>

Scenario:
On vaadin.com we have tabbed layout for Login and Signup dialog. The username field inside the tabs are auto focused. When user switches the tab, the blur event triggers for focused element, resulting into automatic validation, and showing the error message. I would like this automatic validation not to trigger, instead I would user .validate() method on the field on submission of form.

[jouni]

Similar to https://github.com/vaadin/vaadin-text-field/issues/16