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

Issue #77245 by drpal, tedbow, nod_, phenaproxima, Wim Leers, googletorp, rteijeiro, vineet.osscube, tim.plunkett, idflood, joelpittet, pk188, lauriii, BarisW, lokapujya, chr.fritsch, droplet, andrewmacpherson, dmsmidt, dawehner, alexpott, jessebeach, NickWilde, DuaelFr, Cottser, seutje, samuel.mortenson: Provide a common API for displaying JavaScript messages

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

Issue #2476947 by mdrummond, Wim Leers, davidhernandez, lauriii, joelpittet, andypost, Cottser, JeroenT, Manuel Garcia, rpayanm: Convert "title" page element into a block

Issue #2512062 by dsnopek, tim.plunkett, Wim Leers: VariantInterface extends ConfigurablePluginInterface so PageDisplayVariantSelectionEvent should allow passing configuration to the Variant (to enable Panels Everywhere)

Issue #2289917 by Wim Leers, mdrummond, lauriii, Manuel Garcia, emma.maria, Scionar, davidhernandez: Convert "messages" page element into blocks

Issue #2309889 by tim.plunkett, dawehner: Fixed DrupalKernel requires a Drupal\Core\*\Plugin directory for annotated classes to be discovered.