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

Issue #2063373 by mondrake, claudiu.cristea, larowlan, arunvs: Fixed Cannot save image created from scratch.

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

Issue #2096703 by mondrake, tim.plunkett | tstoeckler: Image toolkits should use PluginFormInterface and ContainerFactoryPluginInterface.

Issue #2272481 by herom, ParisLiakos, tim.plunkett: Remove usages of watchdog() from forms, plugins and controllers.

Issue #2211227 by mondrake, xjm | fietserwin: Refactor image and imagetoolkit: isExisting, isSupported, supportedTypes, getMimeType.

Issue #2217783 by mondrake: Remove $extension and $mimeType from Image, use image_type_to_xxx function in the getters instead.

Issue #2103621 by mondrake | fietserwin: Move GD logic from ImageInterface to toolkit, tie toolkit instance to Image instance, toolkits should no longer be instantiated separately.

Issue #2053153 by tim.plunkett, dawehner, damiankloip, aspilicious: Allow contrib modules to provide plugins on behalf of optional modules.

Issue #2048827 by claudiu.cristea, tim.plunkett: Move Image toolkit API from system.module to Drupal\Core.