Blue - v1.8.3

Control components: Add innerRef prop

This update adds a new innerRef prop for control based components to
allow access to their respective DOM nodes.

innerRef is the prop name for this convention, established by several
libraries, especially the CSS-in-JS ones.

Example

<Button innerRef={node => this.buttonNode = node}>...</Button>

For integration reasons, I decided to keep the preference ref props, if
defined (like buttonRef or inputRef).

Tests have been added for innerRef checking, and docs have been
updated.