Consistently use ref callback for accessing component DOM element

[stomita]

Currently there are ways of accessing DOM element in the component. For instance, the xxxRef callback approach is used in Input component to retrieve the <input> element, but DateInput uses undocumented instance property of this.input to access its inner <input> element.

To bring consistency, every component should only have reference callback for accessing inner DOM elements, not instance property.

This change will make corruption to existing app which refers DOM element instance property, the deprecation should be done in major update.