Issue #2459819 by Wim Leers, rteijeiro: Remove CacheableInterface (and no longer let block plugins implement it)

Issue #2444231 by Wim Leers, nlisgo: Fix CacheableInterface so it makes sense (was: "Make Config objects & Entities implement CacheableInterface + add BubbleableMetadata::createFromCacheableObject()")

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2432837 by Wim Leers, Fabianx: Make cache contexts hierarchical (e.g. 'user' is more specific than 'user.roles')

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

Issue #2446995 by tim.plunkett, Berdir: Block content titles are not escaped on new block form (Port SA-CONTRIB-2013-082)

Issue #2329101 by Wim Leers: CacheableInterface only has a getCacheKeys() method, no getCacheContexts(), leads to awkward implementations

Issue #2318437 by Wim Leers, Gábor Hojtsy, geertvd: Replace the hardcoded langcode key on blocks with the 'language' cache context

Issue #2342057 by dawehner, hussainweb, Wim Leers: Expand BlockPluginInterface to take into account $return_as_object

Issue #2349991 by fago, Xano, amateescu: Provide a trait for categorizing plugin managers and use it for conditions and actions

Issue #2339151 by EclipseGc, tim.plunkett, Gábor Hojtsy, effulgentsia: Conditions / context system does not allow for multiple configurable contexts, eg. language types

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

Issue #2246647 by mpdonadio, cilefen, YesCT, tim.plunkett: Rename PluginBag to LazyPluginCollection.

Issue #2340123 by Wim Leers: Setting cache tags can be tricky: use strings instead of nested arrays to improve DX.

Issue #2335659 by tim.plunkett, dawehner, effulgentsia, sun: Remove FormState ArrayAccess usage from core.

Issue #2318281 by tim.plunkett, dawehner | kim.pepper: Provide a standard way to check a plugin is an instance of the required interface.