popover: set priv->window before setting doing add_popover() on it

popover: set priv->window before setting doing add_popover() on it

If the popover's relative-to widget is unparented/reparented, we end
up unparenting/reparenting the popover as well. In that case, at the
moment of reparenting, the widget might have been visible (and is
thus mapped again), but priv->window hasn't been set yet.

We must first set priv->window, and then call gtk_window_add_popover(),
that way gtk_popover_map() has its prerequisites straight.

https://bugzilla.gnome.org/show_bug.cgi?id=766323