Add option to use groups as accordion headers

Okay, I played around a bit, but this won't be too easy to solve.

The point is, that IF the setting (grouping_accordion_title) IS enabled, we may not do all the transformations in ...-view-fields.html.twig and instead should just use the regular version there. Instead we have to use {{ title }} as accordion title and the row content as accordion content in views-accordion-foundation-view.html.twig.

So presumably it might be better to solve parts in PHP by switching the themes / theme suggestions if the option is set and use totally different theme files then. That would also make the twig files easier to read.

For now I'll postpone this, until we really need it.

Just for the records: Ran into this again in the next project... not that unimportant feature ;)