Provide a way to plugin to know added and deleted DOM elements for blocks when rewrite with Content Model

[JiuqingSong]

We can find a way to pass the added and deleted DOM element for blocks when rewrite with Content Model.

For example, in editor we have:

1

2

3

And in Content Model we have:
Paragraph1: cachedElement = div 1
Paragraph2: cachedElement = undefined
Paragraph3: cachedElement = div 3

When rewrite, div1 and div3 will not be changed, we need to delete div2 and create a new DIV. After that, we can pass the old div2 and new DIV to plugins so they can do necessary actions if they have some DOM elements cached in the plugins.