Fixed impossible unset of buttons

It is impossible to add your own button because the MCW always checks for `array_intersect_key()` (which doesn't make sense together with `array_merge()` anyway here because then the only option would be to change the icon) although the JavaScript was explicitly to be extended by any button you like :-)

I understand that `array_intersect_key()` makes sense if you want to make sure that the operations are limited to copy, up, down and delete because that's the only thing the MCW handles for you. But I think if a developer sets the buttons himself, it's also his responsibility to make sure those buttons work, isn't it? :-)

I only want to display the sorting buttons so I set
```
'buttons'           => array('up'=>'up.gif', 'down'=>'down.gif')
```

but the current code merges the passed in buttons with those that already exist --> so nothing happens :D