Add slider to Graphics.Input

Not entirely sure how Input stuff works, so please correct me if I got anything wrong (esp. in Native).

Here is a somewhat more complicated test case where you can edit the upper bound of the slider live:

```haskell
import Graphics.Input as Input
import Graphics.Input.Field as Field
import String

ip = Input.input 0

mc : Input.Input Field.Content
mc = Input.input Field.noContent

field = Field.field Field.defaultStyle mc.handle id "Max" <~ mc.signal

s m v = let ma = 
          case String.toFloat m.string of
                    Just f -> f
                    Nothing -> 5
        in Input.slider ip.handle id -5 ma 0.5 v

main = (\x y z -> x `above` y `above` z)
     <~ field ~ (lift2 s mc.signal ip.signal) ~ lift asText ip.signal
```