Issue #3420984 by godotislate, andypost, smustgrave, larowlan, alexpott: Convert Layout DisplayVariant, PageDisplayVariant discovery to attributes

Revert "Issue #2852463 by mikelutz, tim.plunkett, heddn, xjm, MegaChriz, joachim, phenaproxima, dawehner, benjifisher: Create a trait to implement \Drupal\Component\Plugin\ConfigurableInterface"

This reverts commit 109dfd58.

Issue #2852463 by mikelutz, tim.plunkett, heddn, xjm, MegaChriz, joachim, phenaproxima, dawehner, benjifisher: Create a trait to implement \Drupal\Component\Plugin\ConfigurableInterface

Issue #2776975 by joelpittet, dawehner, tim.plunkett, xjm, pfrenssen: March 3, 2017: Convert core to array syntax coding standards for Drupal 8.3.x RC phase

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2569083 by dsnopek, tim.plunkett, Wim Leers: Allow passing cacheable metadata to display variants (to enable Panels Everywhere)