notebook: Reallocate tab gadgets when we reorder them

Otherwise they stay in their position and don't cause a redraw. Which
really confuses Epiphany users apparently.

https://bugzilla.gnome.org/show_bug.cgi?id=762720

gtk/gtknotebook.c

@@ -7586,6 +7586,7 @@ gtk_notebook_child_reordered (GtkNotebook     *notebook,
                              gtk_css_gadget_get_node (page->gadget),
                              sibling);
   gtk_notebook_update_labels (notebook);
+  gtk_css_gadget_queue_allocate (priv->tabs_gadget);
 }
 
 static void