Two ChoiceBox API additions

[Paru Somashekar]

The controls team is planning to add 2 new APIs to ChoiceBox control. 
They are :-

1. valueProperty : (getValue(), setValue() and valueProperty())
The value of a ChoiceBox is defined as the selected item in the 
ChoiceBox selection model. The valueProperty is synchronized with the 
selectedItem, in that when a user sets the value property the selected 
item/index is updated accordingly. And when selection changes, the value 
property also reflects the change. This property allows for 
bi-directional binding of external properties to the ChoiceBox and 
updates the selection model accordingly.

2. converterProperty : (getConverter(), setConverter(), converterProperty())
Allows a way to specify how to represent objects in the items list. When 
a StringConverter is set, the object toString method is not called and 
instead its toString(object T) is called, passing the objects in the 
items list. This is useful when using domain objects in a ChoiceBox as 
this property allows for customization of the representation. Developers 
can set any of the pre-built Converters available in the 
javafx.util.converter package.

thanks,
Paru.