onClick closure; when created in loop, only last item is bound.

```nim
import nigui

app.init()

var window = newWindow()
window.width = 250
window.height = 300

var mainContainer = newLayoutContainer(Layout_Vertical)
var ll = ["foo", "baa", "baz"]
for but in ll:
  var control = newButton(but)
  control.widthMode = WidthMode_Expand
  control.onClick = proc(event: ClickEvent) =
    echo but
  mainContainer.add(control)

window.add(mainContainer)

window.show()
app.run() 
```

https://nim-lang.org/docs/system.html#closureScope was mentioned in the irc so i tried various 
combinations:

```nim
  var control = newButton(but)
  control.widthMode = WidthMode_Expand
  closureScope:
    control.onClick = proc(event: ClickEvent) =
      echo but
  mainContainer.add(control)
```

```
  control.onClick = proc(event: ClickEvent) =
    closureScope:
      let foo = but
      echo foo
  mainContainer.add(control)
```

etc. *always the last sequence item is echoed.*

Please be aware that the goal is not to get the button description (it only happens to be the same as the `but` in this example.)