relative annotations

[eroux]

Having relative annotations coordinates (ie: offset with the previous annotation) would greatly help when editing a portion of an OPF, and would reduce diffs when the base layer is changing. The system can be relatively simple, but requires the layers to contain a list of annotations instead of a dictionary as it is encoded now

I saw the idea in http://multiversiondocs.blogspot.com/2011/02/standoff-markup-explained.html ("Absolute versus relative offsets")