once-displayed

I often find myself writing:
```JavaScript
if(this.is_displayed) {
   // do something which requires the dom to be populated
} else {
    this.on('displayed', function() {
        // do the same thing
    });
}
```
Moreover, the variable `is_displayed` is lacking in general widgets. The new function `once_displayed` is a helper for that. 
It also helps simplifying container widgets.