Issue #2220929 by keithm: Changed ViewfieldWidgetSelect::validateElement() doc to reflect intention.

Revert "Issue #2220929 by keithm: Changed field settings form, putting allowed views and allowed display types inside details element."

This reverts commit e4551e52.

Issue #2220929 by keithm: Changed field settings form, putting allowed views and allowed display types inside details element.

Issue #2220929 by keithm: Assign entity field labels and render single-valued form settings inside a fieldset.

Issue #2220929 by derhasi, Lukas von Blarer, pferlito, frob, daylioti, sachbearbeiter, 4aficiona2, seanpb, nerdacus, keithm, et al.: Viewfield Drupal 8.x initial commit.

Many thanks for the considerable effort of many individuals who contributed to the 8.x Viewfield design.