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.
`valueLoaded` -> `valueGot`, to be consistent with the term `getter`, instead of adding yet another concept/term (`loading`). `valueCached` would be also be more consistent, but than might be confusing together with `cachedValue`. Or `getter` could be renamed to `loader`.

Please also support lower-brained non-native speakers like me. I see the terms `load`, `property`, `value`, `cache`, `fetch`, `request`, `get`... All something different? Well, at least `fetch`, `get` and `load` mean the same, no? Then please stick to one of them, so code reader do not have to hope all the terms really mean the same.