Add App#gutter

Here's an implementation of `#gutter` that addresses #540. I tried to use the reported size of the shell's `ScrollBar`, but since the call to `App#gutter` happens before the shell is opened, the `ScrollBar` said its width was 0. If I call `#gutter` from a click block, or in a spec, I get 16.

In Shoes 3, on OS X, the value of `#gutter` is always 15, whether there is a scrollbar or not. So, at least for now, I'm considering setting the value of #gutter statically to 16. But first, let's see what the value is on other platforms. 

Try running this app on your platform (Windows, Linux) and see what happens when you click the button:

```ruby
Shoes.app width: 100, height: 200 do
  stack do 
    title "Scrollbar"
    button "Gutter" do
      @gutter_size.replace "Gutter: #{gutter}"
    end
    @gutter_size = para "Gutter: #{gutter}"
    para "Round peg, square hole" * 200
    para "Gutter: #{gutter}"
  end
end
```

Thanks!