Add Graphics.Element.scrollContainer

This is a proposal to add the capability to create scrolling regions in UIs built using the `Graphics.Element` API.  

With this change, here is how you would make a 200x300 region containing a vertically-scrolling block of text:

```elm
main = plainText "my text..." |> width 200 |> scrollContainer 200 300 
```

As far as I know, there is currently no way to achieve this type of effect with the core API.