[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.

I mean overridden, not executed. I am talking about behavior, not performance. The way it's written right now is per target. I'm suggesting it not be per-target but instead per-annotation.