columnview: Clear the sorter first thing in dispose

Otherwise, when removing the columns, each column will trigger a
sorter::changed signal emission.

And because sorters are often still connected to a sortlistmodel, we
can't skip that emission and need to do it.
But we only need to do it once.

gtk/gtkcolumnview.c

@@ -477,6 +477,8 @@ gtk_column_view_dispose (GObject *object)
 {
   GtkColumnView *self = GTK_COLUMN_VIEW (object);
 
+  gtk_column_view_sorter_clear (GTK_COLUMN_VIEW_SORTER (self->sorter));
+
   while (g_list_model_get_n_items (G_LIST_MODEL (self->columns)) > 0)
     {
       GtkColumnViewColumn *column = g_list_model_get_item (G_LIST_MODEL (self->columns), 0);