Initial proof of concept

[instification]

Main concepts/tasks:

• Register dynamic vocabularies as persistent utilities
• Subclass AjaxSelectWidget to add supermodel attribute support
• Ensure clean uninstall
• Create plone.app.querystring registry entries (for volto search block query/facet support)
• Create a readme

[djay]

the more I think about it, indexvocabularies doesn't really make sense.
What it does is either uniquevaluesvocabularies or addabletagvocabularies but I think dynamicvocabularies is the best name.

[stevepiercy]

Some grammar, syntax, and style suggestions. Otherwise looks good. Thank you!

[stevepiercy]

reStructuredText uses double backticks for inline literals, whereas Markdown uses single backticks.

Suggested change

	- Use the `plone.app.vocabularies.Catalog` vocabualary. See `this discussion <https://community.plone.org/t/widget-parameter-for-catalogsource-based-choicefield/18129/3>`_
	- Use the ``plone.app.vocabularies.Catalog`` vocabualary. See `this discussion <https://community.plone.org/t/widget-parameter-for-catalogsource-based-choicefield/18129/3>`_