NSDoc / showsProxyIcon implementation

I added a simple property that handles drawing of the document proxy icon in NSDoc apps. In your current implementation the proxy icon was still drawn if you set 'showsTitle' to NO. Drawing of the proxy icon needs to be enabled by setting 'showsProxyIcon' to YES, the default value is NO for now. Thought it would make more sense that way. Documentation is updated as well. You're welcome :)