[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.

> So, given that we can do either, which should we do, in your opinion?

In my opinion, the question has no real meaning until we start to really support multi-target.
When we will do that, we will probably end up reviewing all previous features anyway 
 - so, for now, I would not care about it.

I think the template code would became a bit more complicated if we went with the per-annotation option, so in order to reach maximum simplicity, I vote for keeping the per-target approach.