[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.

> I'll add that a conditional assignment in my code example would completely solve my concern about updates, too.

How about the conditional assignment in the existing code?

Is there something that resets the `@annotation` object, and thus kills the flag set there? We only `angular.extend` `@annotation` with the model, never overwrite it directly, so I think the flag should be preserved ... and according to my tests, it is.