Group up style information changes and report at the end of an op batch

Operation batches that modify a large number of paragraphs (or repeatedly modify the same paragraph) can cause numerous unnecessary style change events to be triggered. Rather than reporting these instantly, track them as they occur and emit the style change summary on batch end.

Related to #610.
The call can be completely eliminated if desired. It was always emitting a signal, I haven't changed that feature at all. All I've done here is make it more obvious right?