Reviewed by Beth.

        Some more clean-up of AccessibilityObject:
        - Rearranged/grouped methods.
        - Marked many methods const.
        - Changed many methods to take args by const ref instead of by val.
        - Made headingLevel() static.
        - Renamed detached() to isDetached(), loaded() to isLoaded(),
          documentLinks to getDocumentLinks().
        - Made clearChildren(), removeAXObjectID(), isDetached() private.
        - Added notImplemented() calls for unimplemented methods.
        - Replaced stringForReplacedNode method with static function.




git-svn-id: http://svn.webkit.org/repository/webkit/trunk@31649 268f45cc-cd09-0410-ab3c-d52691b4dbfc