Change window location when popups are block

In the event of window.open being stopped by a popup blocker the message handler now changes the window.location, thus providing graceful degradation of navigation messages.
I would set a singleton object instead of the class, which right now seems to be `new`ed every time a 'navigate' event comes in:

```coffee
windowFacade = new WindowFacade()
CT._WindowFacade_ = windowFacade
...
  windowFacade.displayExternalURL url
```