Add the option to set the disabledColor of an FUIButton

Add the option to set the `disabledColor` (and `disabledShadowColor`) of an `FUIButton`.

- If the `disabledColor` is not set, do nothing and allow the default behaviour of `adjustsImageWhenDisabled` to happen.
- If `disabledColor` is set but `disabledShadowColor` is not, `disabledShadowColor` defaults to `shadowColor`.
- If `disabledColor` is not set, setting `disabledShadowColor` has no effect.

(Similar to [pull request #121](https://github.com/Grouper/FlatUIKit/pull/121) - Add the option to assign a highlightedColor to FUIButton)