Other controls are added to the map-overlay if they are added after the OverviewMap Control

### This issue is for a: (mark with an `x`)
```
- [ x ] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
- Add an `OverviewMap` control to the map
- Add another type of control to the map

The secondly added control is added inside the overlay of the `OverviewMap` control

### Mention any other details that might be useful

This only affect the controls added **after** the `OverviewMap` control. If the `OverviewMap` control is the last added, everything works fine.

```
//Wait until the map resources are ready.
map.events.add('ready', function () {
// The compass is added on the top-right corner of the map
map.controls.add(new atlas.control.CompassControl(), {
position: 'top-right'
});

//Add the bring data into view control to the map.
map.controls.add(new atlas.control.OverviewMap(), {
position: 'top-left'
});

// The style control will be added to the bottom-right corner of the overlay
map.controls.add(new atlas.control.StyleControl(), {
position: 'bottom-right'
});
});
```