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.
Reason why I had my issues with `emitStylingChanges` is e.g. here. It sounds strange to emit a signal here in the middle of the constructor. Could this not just directly set the respective variables, instead of trying to reuse existing methods?