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.
Merge pull request #648 from peitschie/style-change-evt

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