We can only use `legalize_for_universal_path` for base-name
paths relative to the Ardour session folder.
The full absolute path can contain locally legal chars,
which legalize_for_universal_path would replace, so we need
to be careful.
We should also use `legalize_for_universal_path` for the
snapshot name (and name used in interchange), but that may break
sessions.
228be635ee