[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.

Good point. But IIRC, `$watchCollection` does a shallow check, so it won't pick up changes inside the targets. Maybe calling `$watch` with the optional `objectEquality` flag would be a better solution? ([Quick link to docs](https://docs.angularjs.org/api/ng/type/$rootScope.Scope))