Additional Custom Attribute Autocomplete Constraint

[sbatchelder]

The ability to autocomplete str type custom attributes based on a LeafType is good, but can still allow for errors or mispellings. It also doesn't allow for a dropdown of possible valid options to be viewed during assignment.

I propose either

1. an additional constraint option that prevents saving (shows an error) if the entered text is does not have a LeafType Leaf match.
2. A new enum type/option that sources the enum values directly from a LeafType, where the enum value is the leaf path and the enum label is the leaf name.