allow image widgets to do alpha-blitting

when the image (e.g. a PNG) does contain an alpha channel, that can
be honored by ImageWidget. It doesn't do so by default for compatibility
(arguably, we should change that in the future), it has to be enabled
by setting the "alpha" property to "true" (boolean, not string).
honor alpha when painting the dog-ear bookmark icon

allows for different icons making use of that. For our current icon,
it actually doesn't.

Fixes #402