Fix importing the publisher class

Use the new name directly to fix errors like this one:

Traceback (most recent call last):
  File "./PoPiPaint.py", line 22, in <module>
    import CanvasFrame
  File ".../CanvasFrame.py", line 22, in <module>
    import CanvasModel
  File ".../CanvasModel.py", line 21, in <module>
    from wx.lib.pubsub import Publisher as pub
ImportError: cannot import name Publisher