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

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 #2363025 by Wim Leers, dawehner, larowlan: Push usage of drupal_set_page_content() higher: out of blocks and page display variants.