add support for transparent windows

[tstriker]

in do_expose we can do this trick to paint the background transparent:

        context.set_operator(cairo.OPERATOR_CLEAR)
        context.fill_preserve()
        context.set_operator(cairo.OPERATOR_OVER)

the main app window also needs this: win.set_app_paintable(True) win.set_colormap(win.get_screen().get_rgba_colormap())

via http://stackoverflow.com/a/4902541/46617