Open Radar

 
UIButton would benefit greatly from proper support for different tint colors per UIControl state.

A very common design for buttons is a button with text and an image. UIButton provides the support for this with `image(for:)` and `setImage(for:)`. However, while it is possible to change the title color per state, it's not possible to change the image's tint color per state. If this were possible, it would enable designs where the image tint color follows the title color without having to resort to overriding all the state setters.

For example, one could make a button with a title, template image and background image as follows;

* Normal state: Orange background image, black title text and black icon.
* Disabled state: Gray background image, white title and white icon.

Currently, this requires to set separate images per state, instead of being able to rely on the tint color.

Proposed API:

```
open func UIButton.setTintColor(_ color: UIColor, for controlState: UIControl.State)
open func UIButton.tintColor(for controlState: UIControl.State) -> UIColor?
```