NB: get this log message when removing a binding:
Gtk-CRITICAL **: 00:19:47.333: gtk_tree_model_filter_get_path: assertion
'GTK_TREE_MODEL_FILTER (model)->priv->stamp == iter->stamp' failed
But it doesn't really seem to be CRITICAL.
Note: ideally we should subscribe to settings changes and update the TreeModel
in the listener instead of updating it directly in the signal handler for the
accel editor.