Implements classes Annotation, AnnotationBorderStyle, AnnotationLayer, AnnotationHandler and LinkAnnotation
Creates unit tests for the annotation layer
Cleans up viewer.css
Adds annotation constants to src/shared/util.js
Updates web/page_view.js to only make it responsible for printing the annotations and not for styling them
Rewrites AnnotationUtils to simplify the code and to make it work with the new annotation classes
Rewrites src/core/worker.js and src/core/core.js to simplify fetching annotations