Add method to ElementLinearData to get IVStore items in use

This is minimise the amount of data we need to serialise when
sending this over the wire.

The minimal IVStore data is added to the MW bug report, and
editedData fixed to only return the data array, not the full
LinearData object.

Documentation in AnnotationSet has finally been updated to
refelect the fact that it only stores Annotations
(was previous the generic OrderedHashSet).

getAnnotationFromOffset has been split out into a function
that just returns this indexes so that in cases where we
don't need the values we don't do an unneccesary store lookup.

Bug: 47318

Change-Id: I4819cf06d1bd0ae4f8b896052e278ca75c9551bf
Merge "Add method to ElementLinearData to get IVStore items in use"