Issue #2027423 by claudiu.cristea, tim.plunkett, andypost: Make image style system flexible.

core/modules/image/image.admin.inc

core/modules/image/image.api.php

core/modules/image/image.module

core/modules/image/lib/Drupal/image/ImageStyleInterface.php

.../image/lib/Drupal/image/Plugin/Core/Entity/ImageStyle.php

core/modules/image/lib/Drupal/image/Tests/FileMoveTest.php

...les/image/lib/Drupal/image/Tests/ImageAdminStylesTest.php

...ules/image/lib/Drupal/image/Tests/ImageDimensionsTest.php

...es/image/lib/Drupal/image/Tests/ImageFieldDisplayTest.php

...dules/image/lib/Drupal/image/Tests/ImageFieldTestBase.php

...ules/image/lib/Drupal/image/Tests/ImageStyleFlushTest.php

...mage/lib/Drupal/image/Tests/ImageStylesPathAndUrlTest.php

...s/image/lib/Drupal/image/Tests/ImageThemeFunctionTest.php

...ture/lib/Drupal/picture/Tests/PictureFieldDisplayTest.php

core/modules/picture/picture.module

...les/rdf/lib/Drupal/rdf/Tests/ImageFieldAttributesTest.php

.../modules/rdf/lib/Drupal/rdf/Tests/StandardProfileTest.php

core/modules/rdf/rdf.module