Change heuristic for when to incrementally update
* Only incrementally update if the added/removed text doesn't contain
  any characters that could change the DOM structure.
* Fixed up some test expectations for edits that used to be incremental
  but aren't now (specifically, adding/deleting attributes).
Merge pull request #4863 from adobe/nj/safer-incremental

[LiveHTML] Change heuristic for when to incrementally update