Create HtmlKeyValueListAttributeRenderer

[graste]

The attribute exists in trellis, but we don't have a renderer for it. The ImageList renderer contains a simplified example an how to do this. We need at least a template as_input.twig – a twig template for the collection/hierarchy views would be nice as well.

[graste]

When there are allowed keys, allowed values or even allowed pairs being specified the restriction could lead to changing the text inputs to single selects for the restricted sets. With and w/o javascript two text inputs or selects need to be there to allow adding pairs. Later on a javascript widget may be added to improve input convenience via selectize or similar.