While calling `JSON.stringify(...)` on a class-instance obviously "works" (as in it doesn't throw), since it's really just an Object, it doesn't really make much sense in the context of the `AnnotationStorage.hash`-getter.
Also, access the *inverse* Viewport-transform correctly in `FreeTextEditor.serialize` to prevent errors being thrown when that method is invoked.
Finally, slightly updates the `AnnotationStorage.serializable`-getter to improve consistency within the class.
