Hierarchical CRUD does not allow to add new root nodes

[nursix]

S3HierarchyCRUD:
https://github.com/sahana/eden/blob/master/modules/s3/s3hierarchy.py#L46

...allows to add child categories to existing categories, but it doesn't have an option to add new root categories.

While for some use-cases (e.g. branch hierarchy of an organization) this is deliberate, other use-cases (e.g. type hierarchies) would need an option to add new root nodes - so should be extended to support that.

One possible way could be to add a pseudo-root node (labeled with the title_list CRUD string of the entity itself), so that it offers a context menu with (just) "Add". Detection of the special nature of this node in the client-side script could be achieved by mapping it to id=0.