Issue #2035315 by dawehner: Add a dedicated @FieldWidget annotation.

...l/datetime/Plugin/field/widget/DatetimeDatelistWidget.php

...al/datetime/Plugin/field/widget/DatetimeDefaultWidget.php

...b/Drupal/email/Plugin/field/widget/EmailDefaultWidget.php

..._reference/Plugin/field/widget/AutocompleteTagsWidget.php

...tity_reference/Plugin/field/widget/AutocompleteWidget.php

...modules/field/lib/Drupal/field/Annotation/FieldWidget.php

...b/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php

...eld/lib/Drupal/field/Plugin/field/widget/HiddenWidget.php

...Drupal/field_test/Plugin/field/widget/TestFieldWidget.php

...ield_test/Plugin/field/widget/TestFieldWidgetMultiple.php

...eld_test/Plugin/field/widget/TestFieldWidgetNoDefault.php

...s/file/lib/Drupal/file/Plugin/field/widget/FileWidget.php

...mage/lib/Drupal/image/Plugin/field/widget/ImageWidget.php

...s/link/lib/Drupal/link/Plugin/field/widget/LinkWidget.php

...er/lib/Drupal/number/Plugin/field/widget/NumberWidget.php

.../lib/Drupal/options/Plugin/field/widget/ButtonsWidget.php

...ns/lib/Drupal/options/Plugin/field/widget/OnOffWidget.php

...s/lib/Drupal/options/Plugin/field/widget/SelectWidget.php

...xonomy/Plugin/field/widget/TaxonomyAutocompleteWidget.php

.../telephone/Plugin/field/widget/TelephoneDefaultWidget.php