The old version wasn't introspectable as it didn't have a length
return parameter. Also, delete gtk_tree_path_get_indices_with_depth,
since it's no longer needed.
Use ::keynav-failed for arrow navigation in icon views, so that
it is possible to override error handling. Also add API to get the
row/col of an item. With this, it is possible to make arrow keynav
span adjacent icon views, which is desired in the new control-center
shell. testiconview-keynav demonstrates this.
