[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.

Just in case it wasn't clear, I'm not saying "we should do this" I'm saying "should we do this" and asking you're opinion. You've only given me an explanation of what would be required, which I already know, so I presume you're being reluctant because it seems more complicated. In reality, it's quite simple.

So, given that we can do either, which should we do, in your opinion?