[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.


> [...] the showDiff field is being persisted on the save [...]
> 

See my comment here on trying to stop that, using the approach 
suggested by you earlier.