[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.

> IIRC, we have some filtering in place on the bridge to avoid these spilling back to the server. (But I might be wrong.)

Yes, I was wrong. Of course there is no filtering, since we actually need the data in the sidebar. Bah.