[API REVIEW] RT-29812: Add Set<PseudoClass> getPseudoClassState() to javafx.css.Styleable

[David Grieve]

https://javafx-jira.kenai.com/browse/RT-29812

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 Set<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)