adds horizontal text alignment feature for textfields

Textfields offered the possibility of left and right aligned text but not centered. I changed [setRightAligned](http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/scenes/scene2d/ui/TextField.html#setRightAligned-boolean-) for setTextAlignment which makes use of Align class, ensuring only left, center and right options are allowed.

Tested on UITest