Support object oriented approach to styling UI components

The Text class has a setFill method for controlling the color of the text. I suggest this same method should also be part of TextField and TextArea, because no matter if the text is editable or not, the developer should still be able to set the fill color. This issue relates to the other issue I created about adding a setAlignment method to these classes as well. In general I suggest you take a good look at the three classes Text, TextField, and TextArea, and try to make them similar - that will make it much easier for application developers to work with.