Add a dedicated @Editor plugin annotation

Straightforward application of the pattern from #1986988-9: Add a dedicated @CKEditorPlugin annotation.

The only caveat was the necessity of aliasing on of the uses of "Editor" as a class name, since now both the annotation class Editor and the entity class Editor are competing for the same name. Aliasing the entity version made more sense to me and I figured it'd result in less magic than trying to alias an annotation plugin (if that's even possible), which would result in a renamed @EditorAnnotation or something like that.

-use Drupal\editor\Plugin\Core\Entity\Editor;
+use Drupal\editor\Plugin\Core\Entity\Editor as EditorEntity;

#1: editor_annotations-1992744.patch queued for re-testing.

Hm, fighting with testbot Git today.

Ah, damn. It's capitalization that's the problem. I should have seen that coming.

Good enough for RTBC? I'd like to get this in to match #1986988: Add a dedicated @CKEditorPlugin annotation so everything is in the same place for #1879120: Use Drupal-specific image and link plugins — use core dialogs rather than CKEditor dialogs, containing alterable Drupal forms.