[WIP] Resurrect the removed visual diff support

This does what it used to do.
Probably brings back the same concerns which caused it's removal in the past
 - whatever those were.

( @tilgovi ? )

What is missing is the stying of the removed and added parts. (It used to be pasten green and ping bg, but you can get creative on that.

Feedback welcome.

> this will be overridden every time the annotation is updated, 
> [...]
> I don't think you have stopped it.

According to some very simple tests, updating the annotation now longer overrides the value that has been changed manually.