Make a field's order icon white when opened

This makes the field_order dashicon white instead of gray when a field is opened up:

![edit_field_group__van_patten_media__wordpress](https://cloud.githubusercontent.com/assets/1231306/3173428/09f8e042-ebe6-11e3-9874-b902b07932f8.png)

It's just slightly more pretty this way :smile: