popover: pop down when pressing escape

Simply hiding the popover may introduce inconsistencies in application
behavior, since popping up/down is the usual use case.

https://bugzilla.gnome.org/show_bug.cgi?id=773885

gtk/gtkpopover.c

@@ -1461,7 +1461,7 @@ gtk_popover_key_press (GtkWidget   *widget,
 
   if (event->keyval == GDK_KEY_Escape)
     {
-      gtk_widget_hide (widget);
+      gtk_popover_popdown (GTK_POPOVER (widget));
       return GDK_EVENT_STOP;
     }