Enable autoscroll when rubberband selecting

[kbengs]

Autoscroll the view when mouse cursor goes near or outside upper/lower
window edges when rubberband selecting

Iconview actually has built in autoscroll for dnd, rubberband selecting and when navigating with keyboard arrows. It can be enabled with:
-self.sw.add_with_viewport(self.iconview)
+self.sw.add(self.iconview)
(some other small changes are also needed)

The problem is that it wouldn't be possible to have the hack that makes it possible to start a rubberband selection from left margin. (end of on_window_size_request method) And also autoscroll when dnd don't work any more since we have "return True" in iv_dnd_motion metod.
Another positive effect of it would be that iconview height allocation is working correctly..

But because that has negative effects I think this commit is the best way now.