Create a new instance of the default skin for this control. This is called to create a skin for the control if
no skin is provided via CSS -fx-skin or set explicitly in a sub-class with setSkin(...).
Returns: new instance of default skin for this control. If null then the control will have no skin unless one
is provided by css.
This method is called by the assistive technology to request
the value for an attribute.
This method is commonly overridden by subclasses to implement
attributes that are required for a specific role.
If a particular attribute is not handled, the superclass implementation
must be called.
Returns: the value for the requested attribute
Diagram: Choice, Combo