Add NSAttributedString.CursorAttributeName

By adding the _CursorAttributeName_ field to _NSAttributedString_ we gain the possibility to change the mouse pointer shape for specific text regions in _NSTextView_ et al. Who would not want to trick their users like this:

```c#
// muaha.
myTextView.LinkTextAttributes = NSDictionary.FromObjectAndKey (
  NSCursor.OperationNotAllowedCursor,
  NSAttributedString.CursorAttributeName
);
```