http://sourceforge.net/bugs/?func=detailbug&group_id=5470&bug_id=110677

Canvas.CanvasItem & Canvas.Group:
- bind lacks an optional "add" param
- unbind lacks an optional "funcid" param
- tkraise/lower should call self.canvas.tag_XXXX

(markus.oberhumer@jk.uni-linz.ac.at)

Note: I'm *not* fixing "bbox() return value is inconsistent with
Canvas.bbox()" -- it might break existing code.