Implementation of ComparePrevious feature

[Emmacho]

Ability to compare wording as it existed in selected different versions of the document (so additions and deletions, and who proposed them are highlighted.
The feature would work as follows:
-The parties who introduced changes would have colours associated with their changes, and the changes would be marked as to whether they were deletions, additions or replacements, and whether the changes have been approved yet by other parties or not. In international negotiations there may be many colours. To ensure this would still work for the colourblind, the party names would appear if the user hovered the cursor over some text.
-Highlighting of changes could appear on both the earlier and the later version (e.g. an addition in a later version could show as a deletion in an earlier version, to make clear what going back to the earlier version would entail).