[OGo-GNUstep-Port] SkyCollapsibleContent unable to set 'nil' for key: 'visibility'

[Sebastian Reitenbach]

Hi,

> If the element does support 'visibilityDefault' I think its a pretty  
> good idea.
from the SkyCollapsibleContent.m:

/*
   <> visibility           this or visibilityDefault is required
    > visibilityDefault
    > title                default = nil
    > condition            default = NO


Sebastian