Use progressive enhancement for ListWidget

[sweh]

Originally reported by: Florian Pilz (Bitbucket: florianpilz, GitHub: florianpilz)

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We extracted the ListWidget to create a list of elements, which can render a form each using a modal dialog. However the concept greatly differs from gocept.jsform: Rather having good default options for almost anything, the ListWidget requires you to give all information up front. To be more specific: Only fields that have a given label are shown.

We should unify the approach used, so both (gocept.jsform and ListWidget) use progressive enhancements.

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• Bitbucket: https://bitbucket.org/gocept/gocept.jsform/issue/24