#26 disabled & focus bitmaps for ImageButton

[Kevin Altis]

add additional bitmaps to ImageButton to support
various states: disabled, selected, has focus. See the
wxBitmapButton docs.

If 3DBorder is true then wxPython will handle these by
default, but if 3DBorder is false, then the bitmap
displayed never changes.

We will probably want to support additional styles
such as changing the bitmap when the mouse is over the
button.