Add LabelPrefix context to better guess correct input labels

[fzaninotto]

Fixes #7646

Instead of using the resource, the function that builds field and input labels can use the value of a context, which is set by the forms. This allows for better defaults in forms when using <ReferenceInput> and <ArrayInput>.

The change is made BC by keeping the previous behavior when the resource is passed explicitly.

I also took the opportunity to fix default input labels in <SimpleFormIterator> children (instead of resources.posts.fields.aothors.123.name, they are now resources.posts.fields.aothors.name).

[djhi]

SimpleFormIteratorItem should set a prefix as well if I understand correctly, shouldn't it?

[fzaninotto]

SimpleFormIteratorItem should set a prefix as well if I understand correctly, shouldn't it?

No, ArrayInput doesn't change the LabelPrefix, as developers will prefer to have the entire path in the message. For instance:

const PostEdit = () => (
   <Edit>
     <SimpleForm>
       <ArrayInput source="backlinks">
          <SimpleFormIterator>
             <TextInput source="url" />

ArrayInput will generate for the backlinks sources like backlinks.0.url, backlinks.1.url, etc. What the developers need is a constant label for each input, BUT keeping the context of the post: resources.posts.fields.backlinks.url. So we shouldn't try to change the prefix in ArrayInput - this is really designed for ReferenceInputs (and other inputs creating a new ResourceContext).

[djhi]

There's a linter warning left, otherwise 💪