Fix unsetting focus

Make _gtk_window_unset_focus_and_default queue the changes for after the next draw. This achieves two things: first, it avoids invalidating css at the wrong time (e.g. when setting child-visible during size-allocation), and second, it defers the focus change until after the widget is hidden, so that moving the focus has the desired effect of picking a different, visible widget.

Fixes: #3623 (closed)