onActivate/onDeactivate

This seems to be a Flash Player issue (not strictly a FlashPunk one), but Event.ACTIVATE and Event.DEACTIVATE fire twice.

A simple workaround would be to check the last focus event, as in:

```
private function onActivate (e:Event):void
{
    if (_lastFocusEvent === Event.ACTIVATE) return;

    _lastFocusEvent = Event.ACTIVATE;
    ...
}
```
