[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.

> What do you mean "aggregate it across targets"?

If we made "showDiff" an annotation-wide thing (as opposed to a per-target thing), the correct way to handle it would be to set `showDiff` to `true` if there was at least one target for which the quote has changed.