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