acknowledgmentsUseEditor (show the .rtf file in the about window instead of opening in default editor)

This change adds a new property `acknowledgmentsUseEditor`, defaulting to YES, that maintains all of DCOAboutWindow's current behavior.

When set to NO, though, the Acknowledgments button toggles between the normal get-info view and the contents of the designated credits.rtf file without opening the file in an editor.

Unfortunately this meant changes to the Nib in order to add another view, but implementors don't really need to edit the Nib in most cases anyway. Also the view setup code was moved into a private function so that it could be used for both views.

I've been using this change in Balthisar Tidy for the last several months, and thought I'd offer something back.