[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.

Oh, I get it. (It's useful for triggering the displaying of the diff, when we have multiple targets, and global show diff is on, but some targets don't have a diff.)