Add method to FieldtypeFieldsetOpen that returns the child fields in a given template

[Toutouwai]

Short description of the enhancement

When a FieldtypeFieldsetOpen or FieldtypeFieldsetTabOpen field is used in a template, the child fields within the fieldset are indented in the fields list of Edit Template.

But the PW API doesn't provide any simple way to get the fields that are inside a given fieldset in a given template. So this request is to add that. 🙂