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.
Right, it was emitting a signal before already. But that was just a side-effect before. Now it is more a main-feature. And this seems strange now here. So I would prefer if that could be changed, after all `lastReportedSummary` just gets initialized. Which it might not even need to be, as it is used uninitialized right now as well (unless `undefined` is considered a proper init value). Or?