widgets: Make Fx icon invisible when no filters

We use a GimpCellRenderToggle for the Fx icon
in the layer and channel docks. If you hover over it
when there's no filter, a square "checkbox" will appear
and give the impression you can click on it, even though
you currently can not.

This patch connects the "visible" property of the toggle to
the number of filters attached to the drawable, similar to
what we did for "active". This prevents the checkbox from
appearing, and will hopefully reduce user confusion.

app/widgets/gimpdrawabletreeview.c

@@ -238,6 +238,8 @@ gimp_drawable_tree_view_constructed (GObject *object)
   gtk_tree_view_column_set_attributes (column, view->priv->filters_cell,
                                        "active",
                                        view->priv->model_column_filters,
+                                       "visible",
+                                       view->priv->model_column_filters,
                                        NULL);
 
   gimp_container_tree_view_add_toggle_cell (tree_view,