Since not chaining up in gtk_widget_show/gtk_widget_hide is not allowed,
we can't just implicitly delay the hiding in GtkPopover's hide
implementation. Fix this by introducing gtk_popover_popup() and
gtk_popover_popdown() to show or hide a popover with transition and
revert GtkPopover's show/hide implementation to apply their effect
without the transition.
https://bugzilla.gnome.org/show_bug.cgi?id=769706