Auto-strip the path in absolute UNLs for easy feed to find_absolute_unl_node

c.p.get_UNL() returns a node UNL preceeded by its file path.
But when feeding that string to c.viewController.find_absolute_unl_node, it fails, because the file path must be stripped.

This change safely strips the path if found, although still will fail in case the file path / name contains this charcter "#". (very few cases, as opposed as always, as currently is now)

The stripping is done inside the find_absolute_unl_node function.

This makes the resulting string from c.p.get_UNL() directly compatible with c.viewController.find_absolute_unl_node, without the user needing to work on it in most of the cases.