Add public API to get pseudo-class state to javafx.css.Styleable

The Styleable interface comprises the minimal interface required for an object to be styled by CSS, but is currently missing API to get the pseudo-class state of the object.

ADD:
     public ObservableSet<PseudoClass> getPseudoClassStates()

Note that this method is already implemented by Node. Adding it the Styleable interface will allow CSS to avoid casting to Node to get pseudo-class state. CSS will also be able to handle other classes which are Styleable but do not extend Node (e.g., MenuItem, PopupControl, Tab, and TableColumnBase)