Fleshes out ve.dm.Annotation to a class. Annotations in the linear model
will be instances of a subclass of ve.dm.Annotation. Annotations are
defined by subclassing ve.dm.Annotation and registering this subclass
with ve.dm.AnnotationFactory.
ve.dm.AnnotationFactory keeps track of which annotation classes are known,
and has code to match an HTML element to an annotation class, for use in
the converter.
Change-Id: I68802bdb8736ced1f9e04ee49c623944b448141c
