GtkFontButton: Make sure the dialog isn't destroyed on delete-event

The remaining code assumes that the dialog doesn't get destroyed, but
it actually does on delete-event.

In particular, the filter function and the preview text are lost as
soon as the dialog is created and thus when we create a new dialog we
don't have those properties to set on it anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=719936

gtk/gtkfontbutton.c

@@ -1049,6 +1049,9 @@ gtk_font_button_clicked (GtkButton *button)
 
       g_signal_connect (font_dialog, "destroy",
                         G_CALLBACK (dialog_destroy), font_button);
+
+      g_signal_connect (font_dialog, "delete-event",
+                        G_CALLBACK (gtk_widget_hide_on_delete), NULL);
     }
   
   if (!gtk_widget_get_visible (font_button->priv->font_dialog))