Add focused styles for TextField.

I normally would just commit, but since this involves scene2d, I figured I'd give Nate a chance to look at it first.

I just added a background and font color for when the TextField is focused. I also fixed a bug where if a TextField was set to disabled, and it had text, but did not have a disabledFontColor, it would crash.

The ternary is a tad messy now, so it might be better to drop it into an if-else instead, which I will gladly do if we want to do that.