add Selectors section to Registry; move/update imageAPISelector context

[kirschbombe]

This PR:

• creates a new folder/section in the registry for "Selectors"
• edits the old openannotation index page to focus on selectors, not web anno extensions
• copies the current imageAPISelector context from annex to registry/selectors
• updates the imageAPISelector context to point to the current version of image api instead of specific version
• change the ImageApiSelector context to include all selectors - so now it's the Selectors context
• adds note to ImageApiSelector documentation that the default values in the table are for Image 3 and that implementers should consult previous versions for their default values
• includes the selectors context in presentation 3 context so it doesn't need to be imported separately
• update link to selectors registry in presi 3 documentation
• add mention of webanno selectors and FragmentSelector to Selectors registry
• updates change log

Preview: https://preview.iiif.io/api/image-api-selector-update/registry/selectors/
Context preview: https://preview.iiif.io/api/image-api-selector-update/registry/selectors/context.json

[azaroth42]

Context should use scoped contexts to limit the definition of the properties to only the class.... and define the class.

[thehabes]

Hi all, just sharing some research and results from the JSON LD playground today.

We get jsonld.SyntaxError: Invalid JSON-LD syntax; invalid scoped context. from this context as it is now. We have narrowed it down to two issues

1. This context needs a "iiif_selectors":"http://iiif.io/api/__TODO__/#" entry
2. Under ImageAPISelector in the @context array it tries to scope in "http://iiif.io/api/image/#". This needs to be the context.json and so should be "https://iiif.io/api/image/3/context.json" if you need that context in there.

I am happy to report that we produced a test object which combined this context, the IIIF Presentation API 3 context, and the Goereference Extension context which was processed correctly in the JSON-LD playground. It raised a couple other questions that need researched but it is a promising result. Below is what we found out.

Go Here to see the test object that we brute force embedded the contexts into.
See It Process in the JSON-LD playground.

[zimeon]

Minor issue noted with duplicate defn. of [registry-selectors] but otherwise looks good

[kirschbombe]

The presi 3 context is updated to include the selectors context. A big thanks to @thehabes for testing the context and helping me simplify it.