Improve annotation deletion

[luccalb]

When the textof an annotation is deleted, the annotation visually disappears but is still stored in the annotation map with to === from, which is an invalid annotation state.

• either completely delete the annotation from storage on document remapping
• orprovide a callback function like onAnnotationDelete():boolean to be called when changing the document would result in the removl of an annotation, the user could then provide a function to decide if the document change should pass or not (e.g. a modal)