Destroy Layout Widget's Child Widgets

https://www.pivotaltracker.com/story/show/73818908
I changed the LayoutWidgetDestroyer class to accept settings instead of a setting. Now it's...

```Ruby
def initialize(settings)
  @settings = settings
end
```
It could be...
```Ruby
def initialize(setting)
  @settings = setting.owner.settings
end
```
I don't really care either way...