#2860 Expose added and removed block elements during Content Model rewrite

[JiuqingSong]

The goal of this PR is to let plugins know what is added/removed when we rewrite with Content Model. This is done by:

1. Add a new event type rewriteFromModel
2. Add properties in ModelToDomContext to store those added and removed block elements
3. In model handlers, put newly created block elements into modelToDomContext.addedBlockElements
4. In reuseCachedElements, put removed elements into modelToDomContext.removedBlockElements
5. When create editor, pass these elements via EditorReadyEvent
6. Every time editor DOM is changed from formatContentModel, trigger RewriteFromModelEvent with these elements so plugins can handle these 2 events to know what are added and removed

Note that for removed block elements, if there is list, we will only pass OL/UL element but not the elements under it. So if you want to check removed element and do proper handling, you may also need to go deep to these elements to find the elements you are interested in.

Also note that this event only works for content changed triggered from formatContentModel / setContentModel. If content change is done by other ways such as direct DOM access, we don't trigger this event. To be compatible with old plugins (v8) using EditorAdapter, you can handle ContentChangedEvent and check if event.contentModel exists. If it exists, it means this is from formatContentModel and there will be a RewriteFromModelEvent following so you may not need to handle that ContentChangedEvent. Otherwise, it is from old code and there will not be a realted RewriteFromModelEvent.

[juliaroldi]

Which scenario do we need this change?