Groups are not displayed when using a custom ResultFilter

The groups that are added in a custom `ResultFilter`  the `selectItems` array are not displayed, because `display` is set to `none`. It seems that for the groups to be visible the [rebuildResultItems](https://github.com/ArcBees/gwtchosen/blob/master/plugin/src/main/java/com/arcbees/chosen/client/ChosenImpl.java#L1069) function never sets `display` to `list-item` 
