[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.

> These values should be set on the view model or they may be persisted to the server, which is wasteful and strange. 

IIRC, we have some filtering in place on the bridge to avoid these spilling back to the server. (But I might be wrong.)

> If you do this in the render function you can very easily do it on `@annotation.target`.

OK, will check that.