This adds a method to ensure that a value is from a set of valid values
and raises an error message accordingly.

- Define Enum types for each of the relevant cases
- Adapt call places that were doing such things manually

`mapping.get_sequence(...).as_str_list()` is a very common
pattern seen both in plugins and the core. Adding a helper to reduce
the number of operations will make usage smoother

_create_new_file did not use the 'toplevel' parameter, we can therefore
remove it.

A 'clone' operation has an implicit understanding that it is expensive,
which is not the case of a 'copy' operation, which is more usually
a shallow copy.

Therefore renaming to 'clone'

This makes the 'Node' API public, and available for use directly for
plugins.